Intermittent detection during analytical reactions

ABSTRACT

Methods, devices, and systems for performing intermittent detection during analytical reactions are provided. Such methods facilitate collection of reaction data from disparate reaction times. Further, such methods are useful for reducing photo-induced damage of one or more reactants in an illuminated analytical reaction at a given reaction time. In preferred embodiments, the reaction mixture is subjected to at least one illuminated and non-illuminated period and allowed to proceed such that the time in which the reaction mixture is illuminated is less than a photo-induced damage threshold period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/658,576, filed Oct. 21, 2019, which is acontinuation application of U.S. patent application Ser. No. 15/631,602,filed Jun. 23, 2017, now U.S. Pat. No. 10,563,255, which is acontinuation application of U.S. patent application Ser. No. 15/383,965,filed Dec. 19, 2016, now U.S. Pat. No. 9,738,929, which is acontinuation application of U.S. patent application Ser. No. 14/708,603,filed May 11, 2015, now U.S. Pat. No. 9,556,480, which is a continuationapplication of U.S. patent application Ser. No. 14/091,961, filed Nov.27, 2013, now U.S. Pat. No. 9,057,102, which is a continuationapplication of U.S. patent application Ser. No. 12/982,029, filed Dec.30, 2010, now U.S. Pat. No. 8,628,940, which (1) claims the benefit ofU.S. Provisional Application No. 61/099,696, filed Sep. 24, 2008; (2)claims the benefit of U.S. Provisional Application No. 61/139,402, filedDec. 19, 2008; and (3) is a continuation-in-part application of U.S.patent application Ser. No. 12/413,226, filed Mar. 27, 2009, now U.S.Pat. No. 8,143,030, the full disclosures of all of which areincorporated herein by reference in their entireties for all purposes.

This application is also related to U.S. Provisional Application No.61/072,160, filed Mar. 28, 2008, U.S. patent application Ser. No.12/383,855, filed Mar. 27, 2009, now U.S. Pat. No. 8,236,499, and U.S.patent application Ser. No. 12/413,258, filed Mar. 27, 2009, now U.S.Pat. No. 8,153,375, all of which are incorporated herein by reference intheir entireties for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The use of optically detectable labeling groups, and particularly thosegroups having high quantum yields, e.g., fluorescent or chemiluminescentgroups, is ubiquitous throughout the fields of analytical chemistry,biochemistry, and biology. In particular, by providing a highly visiblesignal associated with a given reaction, one can better monitor thatreaction as well as any potential effectors of that reaction. Suchanalyses are the basic tools of life science research in genomics,diagnostics, pharmaceutical research, and related fields.

Such analyses have generally been performed under conditions where theamounts of reactants are present far in excess of what is required forthe reaction in question. The result of this excess is to provide ampledetectability, as well as to compensate for any damage caused by thedetection system and allow for signal detection with minimal impact onthe reactants. For example, analyses based on fluorescent labelinggroups generally require the use of an excitation radiation sourcedirected at the reaction mixture to excite the fluorescent labelinggroup, which is then separately detectable. However, one drawback to theuse of optically detectable labeling groups is that prolonged exposureof chemical and biochemical reactants to such light sources, alone, orwhen in the presence of other components, e.g., the fluorescent groups,can damage such reactants. The traditional solution to this drawback isto have the reactants present so far in excess that the number ofundamaged reactant molecules far outnumbers the damaged reactantmolecules, thus minimizing or negating the effects of the photo-induceddamage.

A variety of analytical techniques currently being explored deviate fromthe traditional techniques. In particular, many reactions are based onincreasingly smaller amounts of reagents, e.g., in microfluidic ornanofluidic reaction vessels or channels, or in “single molecule”analyses. Such low reactant volumes are increasingly important in manyhigh throughput applications, such as microarrays. The use of smallerreactant volumes offers challenges to the use of optical detectionsystems. When smaller reactant volumes are used, damage to reactants,such as from exposure to light sources for fluorescent detection, canbecome problematic and have a dramatic impact on the operation of agiven analysis. In other cases, other reaction conditions may impact theprocessivity, rate, fidelity, or duration of the reaction, includingsalt or buffer conditions, pH, temperature, or even immobilization ofreaction components within observable reaction regions. In many cases,the effects of these different reaction or environmental conditions candegrade the performance of the system over time. This can beparticularly detrimental, for example, in real-time analysis ofreactions that include fluorescent reagents that can expose multipledifferent reactions components to optical energy. In addition, smallerreactant volumes can lead to limitations in the amount of signalgenerated upon application of optical energy.

Further, in the case of sequencing-by-synthesis applications, anadditional challenge has been to develop ways to effectively sequencenoncontiguous portions of a template nucleic acid on a single molecule.This challenge is exacerbated in template nucleic acids that containhighly repetitive sequence and/or are hundreds or thousands ofnucleotides in length, such as certain genomic DNA fragments. Thedifficulty in generating such noncontiguous reads from a single templatehas hampered efforts to construct consensus sequences for longtemplates, for example, in genome sequencing projects.

As such, methods and systems that result in enhanced reactionperformance, such as an increase in processivity, rate, fidelity, orduration of a reaction of interest, would provide useful improvements tothe methods and compositions currently available. For example, methods,devices, and systems that increase reaction performance by, e.g.,mitigating to some extent photo-induced damage in a reaction of interestand/or increasing various other performance metrics for the reactionwould be particularly useful.

BRIEF SUMMARY OF THE INVENTION

In a general sense, the methods provided herein implement intermittentdetection of analytical reactions as a means to collect reliable datafrom times during the reaction that are less or not able to be analyzedif detection is constant throughout the reaction. In particular, certaindetection methods can cause damage to reaction components, and suchintermittent detection allows the damage to be avoided or at leastdelayed, thereby facilitating detection of the reaction at later stages.For example, if a detection method causes a reduction in processivity ofa polymerase enzyme, then intermittent detection would allow datacollection at noncontiguous regions of a template nucleic acid thatextend farther from the initial binding site of the polymerase on thetemplate than would be achievable under constant detection. Further,some detection methods have limits on how much data or for how long atime data may be generated in a single reaction, and intermittentdetection of such a reaction can allow this data to be collected fromvarious stages of a reaction, thereby increasing the flexibility of theinvestigator to spread out the data collection over multiple stages of areaction. In certain aspects, the present invention is particularlysuitable to characterization of analytical reactions in real time, thatis, during the course of the reaction. In certain aspects, the presentinvention is particularly suitable to characterization of singlemolecules or molecular complexes monitored in analytical reactions, forexample, single enzymes, nucleotides, polynucleotides, and complexesthereof.

In certain aspects, the present invention is directed to methods,devices, and systems for obtaining sequence data from discontiguousportions of single nucleic acid templates. The methods generallycomprise providing a monitorable sequencing reaction comprising apolymerase, template, and primer sequence, as well as the various typesof nucleotides or nucleotide analogs that are to be incorporated by thepolymerase enzyme in the template-directed primer extension reaction.Typically, at least one or more or all of the nucleotides or nucleotideanalogs are embodied with a detectable property that permits theiridentification upon or following incorporation. In the context of thepresent invention, the sequence data for a first portion of a templatenucleic acid is acquired during a first stage of the reaction under afirst set of reaction conditions that includes at least one reactioncondition that results in degraded performance of the reaction, but thatmay contribute to the detectability of the nucleotides beingincorporated. During a second stage of the reaction, the degradativeinfluence is eliminated or reduced, which may result in an inability ora reduced ability to obtain sequence data from a second portion of thetemplate nucleic acid, but where the second portion of the templatenucleic acid is contiguous with the first portion. Subsequently, thereaction condition resulting in degraded performance is reinstated andsequence data is obtained for a third portion of the template nucleicacid during a third stage of the reaction, but where the third portionof the sequence is not contiguous with the first portion of thesequence, but is contiguous with the second portion. The elimination orreduction of the degradative influence during the second stage of thereaction may be accomplished by changing or shortening one or morereaction conditions underlying degradative reaction performance, e.g.,by changing one or more reaction conditions (e.g., temperature, pH,exposure to radiation, physical manipulation, etc.), and in particularmay involve altering a reaction condition related to detection of one ormore aspects or products of the reaction. However, in preferredembodiments, nucleotides or nucleotide analogs having the detectableproperty are present in the reaction mixture during all stages of thereaction, including stages in which the degradative influence iseliminated or reduced; as such, the reaction condition changed in stagetwo of such an embodiment would not comprise removal or dilution of suchdetectable nucleotides or nucleotide analogs.

In certain aspects, the present invention is generally directed tomethods, devices, and systems for enhancing the performance ofilluminated reactions. The term “illuminated reactions” as used hereinrefers to reactions which are exposed to an optical energy source. Incertain preferred embodiments, illuminated reactions comprise one ormore fluorescent or fluorogenic reactants. Typically, such illuminationis provided in order to observe the generation and/or consumption ofreactants or products that possess a particular optical characteristicindicative of their presence, such as a shift in the absorbance spectrumand/or emission spectrum of the reaction mixture or its components. Insome aspects, enhancing the performance of an illuminated reaction meansincreasing the processivity, rate, fidelity, and/or duration of thereaction. For example, enhancing the performance of an illuminatedreaction can involve reducing or limiting the effects of photo-induceddamage during the reaction. The term “photo-induced damage” refersgenerally to any direct or indirect impact of illumination on one ormore reagents in a reaction resulting in a negative impact upon thatreaction.

In certain aspects, methods of the invention useful for characterizingan analytical reaction comprise preparing a reaction mixture andinitiating the analytical reaction therein, subjecting the reactionmixture to at least one detection period and at least one non-detectionperiod during the course of the analytical reaction, collecting dataduring both the detection period(s) and the non-detection period(s), andcombining the collected data to characterize the analytical reaction. Incertain embodiments, the analytical reaction comprises an enzyme thatexhibits an improvement in performance as compared to its performance inthe analytical reaction under constant illumination, and suchimprovement may be related to various aspects of enzyme activity, e.g.,processivity, fidelity, rate, duration of the analytical reaction, andthe like. In certain embodiments, stop or pause points are used tocontrol the activity of the enzyme, and such stop or pause points maycomprise elements such as large photolabile groups, strand-bindingmoieties, non-native bases, and others well known in the art. In certainpreferred embodiments, the one or more detection periods are illuminatedperiods and the one or more non-detection periods are non-illuminatedperiods. In certain preferred embodiments, a plurality of analyticalreactions disposed on a solid support are characterized, preferably in acoordinated fashion as described elsewhere herein.

In certain preferred embodiments, the analytical reaction is asequencing reaction that generates sequence reads from a single nucleicacid template during the detection period(s) but not during thenon-detection period(s). For example, the analytical reaction cancomprise at least two or more detection periods and can generate aplurality of noncontiguous reads from the single nucleic acid template.In some embodiments, the single nucleic acid template is at least 100bases in length and/or comprises multiple repeat sequences. In certainembodiments, the sequencing reaction comprises passage of the singlenucleic acid template through a nanopore, and in other embodiments thesequencing reaction comprises primer extension by a polymerase enzyme.

The analytical may optionally be a processive reaction monitored in realtime, i.e., during the course of the processive reaction. In preferredembodiments, such a processive reaction is carried out by a processiveenzyme that can repetitively execute its catalytic function, therebycompleting multiple sequential steps of the reaction. For example, aprocessive polymerization reaction can comprise a polymerase enzymerepetitively incorporating multiple nucleotides or nucleotide analogs,as long as such are available to the polymerase within the reactionmixture, e.g., without stalling on the template nucleic acid. Such aprocessive polymerization reaction can be prevented by incorporation ofnucleotides or nucleotide analogs that contain groups that blockadditional incorporation events, e.g., certain labeling groups or otherchemical modifications.

In certain preferred embodiments, the analytical reaction comprises atleast one component comprising a detectable label, e.g., a fluorescentlylabeled nucleotide. In certain embodiments, the labeled component ispresent throughout the course of the analytical reaction, i.e., duringboth the detection and the non-detection periods. The method may furthercomprise an optical system to collect the data during the detectionperiod, but optionally not to collect the data during the non-detectionperiod.

In certain aspects, methods of the invention comprise providing asubstrate having a reaction mixture disposed thereon and illuminatingthe reaction mixture on the substrate with an excitation illuminationfor multiple, noncontiguous periods during the course of the reaction,thereby subjecting the reaction mixture to intermittent excitationillumination. In some embodiments, the reaction mixture comprises firstreactant and a second reactant, wherein an amount of photo-induceddamage to the first reactant occurs as a result of interaction betweenthe first reactant and the second reactant under excitationillumination. In certain embodiments, the method further comprisesmonitoring a reaction between the first and second reactants duringillumination and collecting the data generated therefrom. In someembodiments, the reaction is a primer extension reaction and/or thefirst reactant is a polymerase enzyme. In certain embodiments, thesecond reactant is a fluorogenic or fluorescent molecule.

In yet another aspect, the methods are useful for mitigatingphoto-induced damage in an illuminated reaction by subjecting theilluminated reaction to intermittent illumination rather than constantillumination. For example, certain methods of the invention monitor areaction mixture comprising at least one enzyme and a fluorescent orfluorogenic substrate for the enzyme, wherein interaction of the enzymeand the substrate under excitation illumination can result in alteredactivity of the enzyme, e.g if such excitation illumination is presentover an extended period of time. Such methods can comprise directingintermittent excitation illumination at a first observation region for afirst period that is less than a photo-induced damage threshold periodunder the intermittent illumination conditions, but that is greater thana photo-induced damage threshold period under constant illuminationconditions. As such, certain aspects of the invention lengthen aphoto-induced damage threshold period for an analytical reaction throughintermittent inactivation of the excitation illumination source sincethe photo-induced damage threshold period under intermittentillumination is longer than the photo-induced damage threshold periodunder constant illumination.

In a related aspect, the invention also provides methods of performingan enzyme reaction, comprising providing an enzyme within a firstobservation region, contacting the enzyme with a fluorescent orfluorogenic substrate for the enzyme, and directing an excitationradiation at and detecting signals from the first observation region fora period that is less than a photo-induced damage threshold period underintermittent illumination conditions, but that is greater than aphoto-induced damage threshold period under constant illuminationconditions.

In further aspects, the invention provides methods of monitoring aprimer extension reaction, comprising providing a polymerase enzymewithin a first observation region, contacting the polymerase with atleast a first fluorescent or fluorogenic nucleotide analog, andmonitoring a fluorescent signal emitted from the first observationregion in response to illumination with excitation radiation for aperiod that is less than a photo-induced damage threshold period underintermittent illumination conditions, but that is greater than aphoto-induced damage threshold period under constant illuminationconditions.

In addition, the invention provides methods for generating a pluralityof noncontiguous sequence reads from a single nucleic acid templatemolecule. Such methods generally comprise preparing a reaction mixturecomprising the template molecule, a polymerase enzyme, and a set ofdifferentially labeled nucleotides or nucleotide analogs, wherein theset comprises at least one type of nucleotide or nucleotide analog foreach of the natural nucleobases (A, T, C, and G). The polymerizationreaction is initiated, the polymerase begins processive incorporation ofthe labeled nucleotides or nucleotide analogs into a nascent nucleicacid strand, and during such incorporation the reaction is monitored byoptical means to detect incorporation events, thereby generating a firstsequence read. In a subsequent step, the labeled nucleotides or analogsare replaced with unlabeled nucleotides or nucleotide analogs and thepolymerization is allowed to proceed without detecting incorporationevents. Subsequently, the unlabeled nucleotides or analogs are replacedwith labeled nucleotides or nucleotide analogs and the polymerization isallowed to proceed once again with real time detection of incorporationevents, thereby generating a second sequence read that is noncontiguousto the first sequence read. The substitution of labeled for unlabeled,and unlabeled for labeled, nucleotides and nucleotide analogs can berepeated multiple times to generate a plurality of noncontiguoussequence reads, each of the plurality generated during a period when thelabeled nucleotides or nucleotide analogs are being incorporated intothe nascent strand and such incorporation is being detected in realtime.

In certain aspects, devices of the invention can comprise a solidsupport (e.g., substrate) having an observation region, a first reactantimmobilized within the observation region, and a second reactantdisposed within the observation region, and a means for subjecting theobservation region to at least one illuminated period and at least onenon-illuminated period. In certain embodiments, interaction between thefirst and second reactants under excitation illumination causesphoto-induced damage to the first reactant, and further wherein thephoto-induced damage is reduced by subjecting the observation region tointermittent illumination. In some embodiments, the first reactant is anenzyme (e.g., a polymerase), the second reactant (e.g., a nucleotide)has a detectable label (e.g., fluorescent label), and/or the observationregion is within a zero-mode waveguide. The means for subjecting theobservation region to one or more illuminated and non-illuminatedperiods may comprise, e.g., a laser, laser diode, light-emitting diode,ultra-violet light bulb, white light source, a mask, a diffractiongrating, an arrayed waveguide grating, an optic fiber, an opticalswitch, a mirror, a lens, a collimator, an optical attenuator, a filter,a prism, a planar waveguide, a wave-plate, a delay line, a movablesupport coupled with the substrate, and a movable illumination source,and the like. The device may further comprise a means for collecting thedata during the illuminated period(s), such as an optical train, e.g.,operably coupled to a machine comprising machine-readable medium ontowhich such data may be written and stored.

In further aspects, the invention provides systems for performingintermittent detection of an analytical reaction comprising reagents forthe analytical reaction disposed on a solid support, a mounting stageconfigured to receive the solid support, an optical train positioned tobe in optical communication with at least a portion of the solid supportdetect signals emanating therefrom, a means for subjecting the portionof the solid support to at least one detection period and at least onenon-detection period, a translation system operably coupled to themounting stage or the optical train for moving one of the optical trainand the solid support relative to the other, and a data processingsystem operably coupled to the optical train. In certain preferredembodiments, the analytical reaction is a sequencing reaction and/or thesolid support comprises at least one zero-mode waveguide.

In still other aspects, the invention provides systems for analyzing anilluminated reaction that is susceptible to photo-induced damage whenilluminated for a period longer than an photo-induced damage thresholdperiod, comprising a solid support having reagents for the reactiondisposed thereon, a mounting stage supporting the solid support andconfigured to receive the solid support, an optical train positioned tobe in optical communication with at least a portion of the solid supportto illuminate the portion of the solid support and detect signalsemanating therefrom, a means for subjecting the portion of the solidsupport to at least one detection period and at least one non-detectionperiod, and a translation system operably coupled to the mounting stageor the optical train for moving one of the optical train and the solidsupport relative to the other. In some embodiments, the illuminatedreaction is a sequencing reaction, e.g., a nucleotidesequencing-by-synthesis reaction. In certain embodiments, the solidsupport comprises at least one optical confinement, e.g., a zero-modewaveguide.

The invention provides methods of performing analytical reactions, e.g.,processive analytical reactions, that include preparing a reactionmixture comprising reaction components, at least one of which is adetectable component that is detectable during one or more detectionperiods, and at least one of which is a clocking component that isdetectable during one or more non-detection periods during theanalytical reaction. The methods further comprise initiation theanalytical reaction and maintaining conditions that allow the analyticalreaction to proceed while subjecting it to at least one detection periodand at least one non-detection period, both in the presence of theclocking component and the detectable component. In certain embodiments,the detectable component emits a detectable signal is response toexcitation illumination during the detection period, but not during thenon-detection period when a clocking signal is emitted from the clockingcomponent. The detectable signal is collected during the detectionperiod and the clocking signal is detected during the non-detectionperiod, e.g., using an optical system. Optionally, the clocking signalcan also be collected during the detection period and the non-detectionperiod. In certain preferred embodiments, detection data is collected inread time during the detection period, non-detection data is collectedin real time during the non-detection period, and the detection data andnon-detection data are both used to characterize the analyticalreaction. In some embodiments, the transition between the detectionperiod and the non-detection period does not involve substitution and/oraddition of reaction components during progression of the analyticalreaction, and in other embodiments the transition does involvesubstitution and/or addition of reaction components, e.g., via areaction mixture exchange. In some preferred embodiments, a plurality ofanalytical reactions are disposed on a solid support, subjected tointermittent illumination, monitored to collect data, and characterizedbased upon the data so collected.

The detectable component and clocking component are typically linked todiscrete molecules in the analytical reaction. For example, thedetectable component can be linked to a first subset of nucleotideanalogs and the clocking component can be linked to a second subset ofnucleotide analogs in the analytical reaction mixture. Alternatively,both the detectable component and the clocking component can be linkedto a single molecule, e.g., a single nucleotide or nucleotide analog, inthe analytical reaction. The detectable component and clocking componentcan both comprise detectable labels (e.g., luminescent, fluorescent, orfluorogenic labels, including, e.g., quantum dots), and in someembodiments, different detectable labels, e.g. having differentabsorption peaks.

In certain preferred embodiments, an analytical reaction performedaccording to the invention comprises at least one enzyme, e.g., apolymerase, ligase, ribosome, nuclease, and/or kinase. In someembodiments, pause or stop points are engineered into the analyticalreaction to control activity of the enzyme. Various aspects of theanalytical reaction can be changed by being subjected to at least onedetection period and at least one non-detection period, such aspectsincluding but not limited to processivity, fidelity, rate, and duration,e.g. of enzyme activity.

In certain preferred embodiments, the analytical reaction is asequencing reaction comprising a single nucleic acid template thatgenerates sequence reads during the detection period by detecting thedetectable component, and does not generate sequence reads during thenon-detection period by suspending detection of the detectablecomponent. Such a sequencing reaction typically comprises at least twoor three detection periods and generates a plurality of noncontiguoussequence reads from the single nucleic acid template. In someembodiments, the template comprises multiple repeat or complementarysequences. In some embodiments, the sequencing reaction comprisespassage of the single nucleic acid or a nascent strand complementarythereto through a nanopore. In some preferred embodiments, thesequencing reaction comprises primer extension by a polymerase enzymeand the detectable component is linked to a nucleotide or nucleotideanalog. In some embodiments, the clocking component is linked to thepolymerase enzyme, and optionally can be a multi-component label, e.g, aFRET label.

In certain aspects, the invention provides methods of mitigatingphoto-induced damage during an illuminated reaction that includepreparing a reaction mixture having first and second reactants, whereinteraction of the reactants under excitation illumination can causephoto-induced damage to the first reactant. The illuminated reaction issubjected to intermittent excitation illumination characterized byperiods of maximal illumination followed by periods of modified but notabsent illumination. The intermittent excitation illumination reducesthe amount of photo-induced damage to the first reactant during theilluminated reaction as compared to the illuminated reaction underconstant maximal excitation illumination, thereby mitigatingphoto-induced damage to the first reactant. In certain preferredembodiments, the illuminated reaction is a primer extension reaction. Incertain preferred embodiments, the first reactant is an enzyme, e.g., apolymerase or ligase enzyme. In certain preferred embodiments, thesecond reactant comprises a fluorescent or fluorogenic molecule. Incertain embodiments, the modified excitation illumination isillumination with a lower intensity excitation illumination than themaximal excitation illumination. In certain embodiments, a set ofillumination sources provides the maximal excitation illumination and asubset of the set of illumination sources provides the modifiedexcitation illumination.

In other aspects, the invention provides a method of sequencing atemplate nucleic acid that includes subjecting the template tomethylation to generate at least one methylated base, subjecting themethylated base to base excision to generate at least one abasic site inthe template, annealing a primer to the template nucleic acid,contacting the template with a polymerase enzyme to promote extension ofthe primer in a template-dependent manner, monitoring the extension ofthe primer in real time to generate a nucleotide sequence readcomplementary to the template, extending the primer until the abasicsite is encountered by the polymerase, at which time the polymerasepauses on the template, and reinitiating primer extension byfacilitating abasic site bypass by the polymerase. The monitoring,extending, and reinitiating steps are repeated until a desired number ofnucleotide sequence reads is generated and collected, and subsequentlyanalyzed to determine the sequence of the template nucleic acid. Incertain embodiments, the contacting step occurs during a detectionperiod or a detection period immediately follows the contacting step. Incertain embodiments, a detection period ends and a non-detection periodbegins prior to one or more pauses of the polymerase on the template. Incertain embodiments, a non-detection period is terminated simultaneouswith or immediately following one or more reinitiation steps. In someembodiments, the reinitiating step comprises introduction of a pyrene tothe polymerase, where the polymerase incorporates the pyrene into thenascent strand opposite and, therefore, “pairing with” an abasic site inthe template. In certain preferred embodiments, the template is circularand the polymerase pauses at the same abasic site multiple times duringthe primer extension reaction. In other embodiments, the method furthercomprises terminating the monitoring when a desired length of thenucleotide sequence read is collected, e.g., by removing or modifyingexcitation illumination. Optionally, the desired length can be less thana length of the template nucleic acid. Additionally, the monitoring canbe reinitiated subsequent to or simultaneous with the reinitiating ofprimer extension.

In yet further aspects, the invention provides a method of performing anilluminated reaction that includes preparing a reaction mixturecomprising multiple optically detectable components that aredistinguishable from one another based upon their individual signalemissions, initiating the illuminated reaction, and maintainingconditions that allow the illuminated reaction to proceed whilesubjecting the reaction mixture to at least one maximal illuminatedperiod and at least one modified illuminated period during theilluminated reaction. In preferred embodiments, at least a portion ofthe optically detectable components are detectable during both themaximal and modified illuminated periods. In certain embodiments, themaximal illuminated period is characterized by a first excitationradiation intensity and the modified illuminated period is characterizedby a second excitation radiation intensity that is less than the firstexcitation radiation intensity. In certain preferred embodiments, all ofthe optically detectable components are detectable during both themaximal and modified illuminated periods, but are distinguishable fromone another during the maximal illuminated period, but are notdistinguishable during the modified illuminated period. In certainembodiments, the maximal illuminated period comprises exposing thereaction mixture to a set of excitation radiation wavelengths and themodified illuminated period comprises exposing the reaction mixture to asubset of the set of excitation radiation wavelengths. In certainpreferred embodiments, all of the optically detectable components aredetectable and distinguishable during the maximal illuminated period,but only a subset of the optically detectable components are detectableduring the modified illuminated period.

In some embodiments, the illuminated reaction is initiated during amodified illuminated period and subsequently subjected to a maximalilluminated period, where data collected during the modified illuminatedperiod is used in the statistical analysis of data collected during themaximal illuminated period. For example, an illuminated reaction that isa polynucleotide sequencing reaction can generate sequence read dataduring a modified illuminated period that is subsequently used toconstruct a sequence scaffold for assembly of sequence read datacollected during a maximal illuminated period. Additionally oroptionally, the illuminated reaction is a template-directed sequencingreaction and sequence read data collected during a modified illuminatedperiod is used to determine a rate of translocation of a polymeraseduring the modified illuminated period.

Some embodiments of the invention comprise performing a plurality ofilluminated reactions, each of which is exposed to the set of excitationradiation wavelengths during the maximal illuminated period, but isexposed to a different subset of the set of excitation radiationwavelengths during the modified illuminated period, such that a distinctsubset of optically detectable components are detectable during themodified illuminated period for each of the plurality of illuminatedreactions. In other words, for two such illuminated reactions, althoughall optically detectable components are detectable during theirrespective maximal illuminated periods, only a subset of the opticallydetectable components is detectable in each reaction, and the subsetdetectable in the first reaction is preferably different from the subsetdetectable in the second reaction.

In certain aspects, the invention provides methods for performingpaired-end sequencing on a single template molecule. In certainembodiments, such a method comprises providing a double-stranded nucleicacid molecule comprising a first terminal portion, an intermediateportion, and a second terminal portion. A first linker ligated to thefirst terminal portion of the nucleic acid molecule connects the 3′terminus at the first terminal portion with the 5′ terminus at the firstterminal portion; and a second linker ligated to the second terminalportion of the nucleic acid molecule connects the 3′ terminus at thesecond terminal portion with the 5′ terminus at the second terminalportion. A template nucleic acid molecule is thereby formed comprisingthe double-stranded nucleic acid molecule with both the first linker andthe second linker ligated thereto. The template molecule is subjected toa sequencing process in which sequence reads are generated for the firstterminal portion and the second terminal portion, but sequence reads arenot generated for the intermediate portion, even if the intermediateportion is processed during the sequencing process, e.g., by apolymerase. In some embodiments, the first linker and second linker areidentical, and in other embodiments they are different from one another,i.e., not identical. In certain embodiments, the first and secondlinkers comprise complementary regions and can be hybridized to oneanother prior to one or both of the ligating steps. In some cases,hybridized linkers that are ligated to the ends of a double-strandednucleic acid molecule are separated prior to subjecting the molecule toa sequencing reaction, and in some cases the hybridized linkers remainhybridized during at least a portion of the sequencing reaction. Forexample, in a template-directed sequencing reaction, a polymerasecapable of strand displacement separates the hybridized linkers as itsequences the template. In certain preferred embodiments, the sequencingprocess comprises at least one detection period (e.g., an illuminatedperiod) and at least one non-detection period (e.g., a non-illuminatedperiod) such that the intermediate portion of the template molecule issubjected to the sequencing process during the non-detection period. Insome embodiments, the template is fragmented after ligation to removethe intermediate portion. The sequencing process can generate redundantsequence data from one or both of the first terminal portion and thesecond terminal portion, and/or can generate sequence data from anadditional portion of the template molecule that is noncontiguous withthe first terminal portion and the second terminal portion. In preferredembodiments, the sequencing process involves circularizing the templatemolecule by separating the complementary strands of the templatemolecule and using the complementary strands in template-directednascent strand synthesis catalyzed by a single polymerase enzyme.Optionally, the template molecule can comprise a primer binding site, aregistration sequence, and/or a synthesis blocking moiety. The primerbinding site, a registration sequence, or synthesis blocking moiety canbe present in one or both of the linkers, or can be located elsewherewithin the template molecule. In some cases, the synthesis blockingmoiety is selected from the group consisting of an abasic site, a nick,a synthetic linker, a non-native nucleotide or analog thereof, a primer,a large photolabile group, a strand-binding moiety, a damaged base, anda modified base. The synthesis blocking moiety can permanently ortemporarily block progression of the sequencing process, e.g., byinterfering with the activity of an enzyme, e.g., a polymerase enzyme.In certain preferred embodiments, the synthesis blocking moiety is anabasic site, e.g., introduced by a DNA glycosylase.

In some aspects, the invention provides methods for generating a nucleicacid construct for analytical reactions. In certain embodiments, such amethod comprises providing a double-stranded nucleic acid moleculecomprising a first terminal portion, an intermediate portion, and asecond terminal portion; providing a first stem-loop linker hybridizedto a second stem-loop linker; ligating the first stem-loop linker to thefirst terminal portion of the nucleic acid molecule, wherein the firststem-loop linker connects the 3′ terminus at the first terminal portionwith the 5′ terminus at the first terminal portion; and ligating thesecond stem-loop linker to the second terminal portion of the nucleicacid molecule, wherein the second stem-loop linker connects the 3′terminus at the second terminal portion with the 5′ terminus at thesecond terminal portion, thereby generating the nucleic acid construct.Optionally, the nucleic acid construct can be subjected to fragmentationafter the ligating of steps c and d, wherein the fragmentation removesthe intermediate portion from the nucleic acid construct and introducestwo double-stranded termini. The method can further include ligating thetwo double-stranded termini to one another. In some embodiments, one ofthe stem-loop linkers comprises a primer binding site, registrationsequence, or a synthesis blocking moiety that is absent from the otherstem-loop linker.

In further aspects, the invention includes a single template nucleicacid molecule comprising a duplex region; a first linker linking terminiat a first end of the duplex region; a second linker linking termini ata second end of the duplex region, wherein a region of the first linkeris complementary to a region of the second linkers. Optionally, thesingle template molecule comprises the first and second linkershybridized with one another. In some embodiments, the duplex region isseparated or melted apart to transform the single template nucleic acidmolecule into a topologically single-stranded, circular nucleic acidmolecule. Further, the invention provides a composition comprising asingle, optically resolvable polymerase enzyme in association with asingle-stranded circular nucleic acid molecule, wherein thesingle-stranded circular nucleic acid molecule comprises first, second,third, and fourth regions, and further wherein the first region iscomplementary to the second region, and the third region iscomplementary to the fourth region, and further wherein the regions areordered on the single-stranded circular nucleic acid molecule asfollows: first region, third region, second region, fourth region.

In still further aspects of the invention, machine-implemented methodsfor transforming nucleotide sequence read data into consensus sequencedata, wherein the nucleotide sequence read data is generated bysequencing a target region of a template nucleic acid multiple times,and the consensus sequence data is representative of a most likelyactual sequence of the template nucleic acid. Such machine-implementedmethods can comprise various steps, such as a) mapping the nucleotidesequence data to a target sequence using a local alignment method thatproduces a set of local alignments comprising an optimal local alignmentand sub-optimal local alignments, b) enumerating the set of localalignments, c) constructing a weighted directed graph wherein each localalignment in the set of local alignments is represented as a node,thereby generating a set of nodes in the weighted directed graph, d)drawing edges between pairs of nodes in the weighted directed graph ifthe pair represents a potential reconstruction of the template nucleicacid, e) assigning weights to the edges drawn in step d, wherein a givenweight for a given edge represents the log-likelihood that a given pairof nodes connected by the given edge is truly a reconstruction of thetemplate nucleic acid, f) finding the shortest path to each node in theweighted directed graph, thereby generating a set of shortest paths forthe weighted directed graph, g) ranking the set of shortest paths todetermine the best assignment, and h) storing the results of steps a-gon a machine-readable medium. In certain embodiments, the steps of themachine implemented methods are performed via a user interfaceimplemented in a machine that comprises instructions stored inmachine-readable medium and a processor that executes the instructions.Also provided are computer program products comprising a computer usablemedium having computer readable program code embodied therein, saidcomputer readable program code adapted to be executed to implement themachine-implemented methods of the invention, and machine-readablemedium on which the results of the method steps are stored. Theinvention further includes a computer program product comprising acomputer usable medium having a computer readable program code embodiedtherein, said computer readable program code adapted to be executed toimplement the above methods.

In certain aspects, the invention provides machine-implemented methodsfor transforming enzyme velocity data from one or more detection periodsinto a distribution of the distance x travelled by an enzyme (e.g., apolymerase) during a time t, where time t occurs during a non-detectionperiod. Such a method comprises, in certain embodiments, developing aprobability model p(v) to describe an observed distribution of enzymevelocities during one or more detection periods; sampling velocitiesfrom p(v); summing and recording the velocities sampled in step b toproduce a sum that is an estimate of x/τcorr; and repeating thesampling, summing, and recording M times to generate a distribution ofsums that are estimates of x/τcorr, with the distribution of sums beingthe distribution of the distance x travelled by an enzyme during a timet. Preferably, at least some of the steps are performed via a userinterface implemented in a machine that comprises instructions stored inmachine-readable medium and a processor that executes the instructions.Optionally, the enzyme is a polymerase enzyme. In some embodiments,multiple enzymes are observed simultaneously and the probability modelp(v) is determined independently for each of the multiple enzymes. Incertain preferred embodiments,

${p(v)} = {\frac{{{f(v)}{p_{enzyme}(v)}} + {\left\lbrack {1 - {f(v)}} \right\rbrack_{p_{a{rray}}}(v)}}{{\int{{f\left( v^{\prime} \right)}{p_{enzyme}\left( v^{\prime} \right)}}} + {\left\lbrack {1 - {f\left( v^{\prime} \right)}} \right\rbrack_{p_{array}}\left( v^{\prime} \right){dv}^{\prime}}}.}$

In further aspects, the invention provides machine-implemented methodsfor transforming enzyme velocity data from one or more detection periodsinto a distribution of the distance x travelled by an enzyme during atime t, where time t occurs during a non-detection period. In someembodiments, the method comprises estimating a distribution of localrates p(v); making independent identically distributed draws ofN=t/τcorr velocities from p(v); summing the velocities; recording thevelocities summed in c) as an estimate of x/τcorr; and repeating b-d Mtimes, e.g., where M is preferably at least 1000. Optionally, p(v) isdetermined using a Hidden Markov Model or the autocorrelation function

$\left\langle {\delta\;{v(t)}\delta\;{v\left( {t + \Delta} \right)}} \right\rangle\text{∼}{{\exp\left( \frac{- \Delta}{\tau_{corr}} \right)}.}$

The invention further includes a computer program product comprising acomputer usable medium having a computer readable program code embodiedtherein, said computer readable program code adapted to be executed toimplement the above methods, as well as a machine-readable medium onwhich the results of the steps of the methods are stored.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides exemplary embodiments of methods for intermittentillumination of analytical reactions, whether illumination is initiatedbefore (left) or after (right) initiation of the reaction.

FIGS. 2A-2B provide an exemplary embodiment of analysis of a pluralityof illuminated reactions using intermittent illumination, includingdepictions of multiple reactions arrayed on a solid support (FIG. 2A)and prophetic data (FIG. 2B) from certain embodiments of the invention.

FIGS. 3A-3C provide an exemplary embodiment of analysis of a pluralityof illuminated reactions on a solid support (FIG. 3A) using intermittentillumination and a mask (FIG. 3B). A graph (FIG. 3C) depicts propheticdata from certain embodiments of the invention.

FIGS. 4A-4B provide additional embodiments of masks for use in themethods of the invention, including a mask that allows illumination ofcolumns of reactions (FIG. 4A) and a mask that allows illumination ofevery other reaction in a row and column (FIG. 4B).

FIGS. 5A-5D illustrate an aspect of the instant invention in whichmultiple samples are analyzed on a single solid support usingintermittent illumination. FIG. 5A illustrates a solid supportcomprising four quadrants, each quadrant containing a different sample.FIG. 5B illustrates a mask design for selective illumination of thesubstrate. FIGS. 5C and 5D demonstrate various positions of the mask onthe solid support.

FIG. 6 provides an illustration of paths in a sequence alignment matrixrepresenting sequencing data from a SMRTbell™ template.

FIG. 7 illustrates a hypothetical directed graph.

FIGS. 8A-8C provide data from single-molecule sequencing-by-synthesisreactions. FIG. 8A provides data from a two-minute interval beginning atinitiation of the reactions, i.e., from 0-120 seconds. FIG. 8B providesdata from a second two-minute interval from 300-420 seconds. FIG. 8Cprovides data from a third two-minute interval from 600-720 seconds.

FIG. 9 schematically illustrates one embodiment of a system for use withthe methods, devices, and systems of the invention.

FIG. 10 provides a graphical representation of rates of polymeraseactivity on different portions of a template nucleic acid during asequencing reaction utilizing intermittent illumination.

FIG. 11 provides a graphical representation of the average rate ofpolymerase translocation over a template nucleic acid during asequencing reaction utilizing intermittent illumination.

FIGS. 12A-12B provide a distribution of the physical coverage of atemplate nucleic acid achieved during a sequencing reaction utilizingintermittent illumination, with FIG. 12A showing mapping to a referencesequence with sequence reads (and portions thereof) that do not map tothe reference excluded and FIG. 12B showing a similar mapping thatfurther includes sequence reads corresponding to insertions in thetemplate that are absent from the reference sequence.

FIG. 13 provides a distribution of the physical coverage provided bysequence reads generated during sequencing reactions utilizingintermittent illumination across an approximately 40 kb template nucleicacid.

FIG. 14 provides a sequence dot plot for an alignment between a sequenceassembly produced as described herein and a reference sequence.

FIG. 15 provides an exemplary illustration of an HMM for modeling asimple “pausing” vs. “sequencing” system.

FIG. 16A shows a sample of velocities drawn from the HMM in FIG. 15 withthe parameters P(S→P)=1/24; P(P→S)=1/11; and p(v)˜Gamma(48,0.25). FIG.16B illustrates a resulting histogram of local velocities. FIG. 16Cprovides an estimated distance traveled during a non-detection period.

FIG. 17 provides an illustrative example of two observed histograms ofdistances traveled during a non-detection period.

FIG. 18 provides an exemplary strategy for selectively reducing the sizeof a duplex fragment within a SMRTbell™ template.

FIGS. 19A-19C provide an illustrative example of nucleic acid templateshaving nicks.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention pertains. All publications mentionedherein are incorporated herein by reference for the purpose ofdescribing and disclosing devices, formulations and methodologies whichare described in the publication and which might be used in connectionwith the presently described invention.

Note that as used herein and in the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a polymerase”refers to one agent or mixtures of such agents, and reference to “themethod” includes reference to equivalent steps and methods known tothose skilled in the art, and so forth. Where a range of values isprovided, it is understood that each intervening value, between theupper and lower limit of that range and any other stated or interveningvalue in that stated range is encompassed within the invention. Theupper and lower limits of these smaller ranges may independently beincluded in the smaller ranges, and are also encompassed within theinvention, subject to any specifically excluded limit in the statedrange. Where the stated range includes one or both of the limits, rangesexcluding either both of those included limits are also included in theinvention.

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features and procedures wellknown to those skilled in the art have not been described in order toavoid obscuring the invention. Although a number of methods andmaterials similar or equivalent to those described herein can be used inthe practice of the present invention, the preferred materials andmethods are described herein.

I. General

In a general sense, the methods, devices, and systems provided hereinimplement intermittent detection of analytical reactions as a means tocollect reliable data from times during the reaction that are less ornot able to be analyzed if detection is constant throughout thereaction. In particular, certain detection methods can cause damage toreaction components, and such intermittent detection allows the damageto be avoided or at least delayed, thereby facilitating detection of thereaction at later stages. For example, if a detection method causes areduction in processivity of a polymerase enzyme, then intermittentdetection would allow data collection at noncontiguous regions of atemplate nucleic acid that extend farther from the initial binding siteof the polymerase on the template than would be achievable underconstant detection. Further, some detection methods have limits on howmuch data or for how long a time data may be generated in a singlereaction, and intermittent detection of such a reaction can allow thisdata to be collected from various stages of a reaction, therebyincreasing the flexibility of the investigator to spread out the datacollection over multiple stages of a reaction. In certain aspects, thepresent invention is particularly suitable to characterization ofanalytical reactions in real time, that is, during the course of thereaction. In certain aspects, the present invention is particularlysuitable to characterization of single molecules or molecular complexesmonitored in analytical reactions, for example, single enzymes,nucleotides, polynucleotides, and complexes thereof.

In certain aspects, the present invention is directed to methods,devices, and systems for obtaining sequence data from discontiguousportions of single nucleic acid templates. The methods generallycomprise providing a monitorable sequencing reaction comprising apolymerase, template, and primer sequence, as well as the various typesof nucleotides or nucleotide analogs that are to be incorporated by thepolymerase enzyme in the template-directed primer extension reaction.Typically, at least one or more or all of the nucleotides or nucleotideanalogs are embodied with a detectable property that permits theiridentification upon or following incorporation. In the context of thepresent invention, the sequence data for a first portion of a templatenucleic acid is acquired during a first stage of the reaction under afirst set of reaction conditions that includes at least one reactioncondition that results in degraded performance of the reaction, but thatmay contribute to the detectability of the nucleotides beingincorporated. During a second stage of the reaction, the degradativeinfluence is eliminated or reduced, which may result in an inability ora reduced ability to obtain sequence data from a second portion of thetemplate nucleic acid, but where the second portion of the templatenucleic acid is contiguous with the first portion. Subsequently, thereaction condition resulting in degraded performance is reinstated andsequence data is obtained for a third portion of the template nucleicacid during a third stage of the reaction, but where the third portionof the sequence is not contiguous with the first portion of thesequence, but is contiguous with the second portion.

The elimination or reduction of the degradative influence during thesecond stage of the reaction may be accomplished by changing orshortening one or more reaction conditions underlying degradativereaction performance, e.g., by changing one or more reaction conditions(e.g., temperature, pH, exposure to radiation, physical manipulation,etc.), and in particular may involve altering a reaction conditionrelated to detection of one or more aspects or products of the reaction.For example, such an alteration in reaction conditions during the secondstage may result in an increase in reaction rates, e.g., speeding up theprogression of a template nucleic acid through a nanopore; or may reduceexposure of reaction components to harmful radiation or other reactioncondition related to detection of the products of the reaction. However,in preferred embodiments, nucleotides or nucleotide analogs having thedetectable property are present in the reaction mixture during allstages of the reaction, including stages in which the degradativeinfluence is eliminated or reduced; as such, the reaction conditionchanged in stage two of such an embodiment would not comprise removal ordilution of such detectable nucleotides or nucleotide analogs.

“Intermittent detection,” as used herein, generally refers to a means ofmonitoring a reaction that is carried out intermittently during thecourse of the reaction. Intermittent detection may refer to intermittentuse of one or more monitoring methods, but does not necessarily meanthat all means of monitoring a given reaction are intermittently halted.For example, monitoring of one or more nucleotide incorporations togenerate nucleotide sequence reads may be intermittently halted whileother aspects of a sequencing reaction are constantly monitored, e.g.,temperature, reaction time, pH, etc. In certain embodiments,intermittent detection is achieved by intermittent or differentialillumination of a given reaction, e.g., a reaction that uses anillumination system to detect reaction products and/or progression.Although various aspects of the invention are described herein in termsof embodiments using intermittent illumination, it should be understoodthat where applicable intermittent detection by other means (e.g.,electrochemical, radiochemical, etc.) can be utilized in the methods ofthe invention. Likewise, a stage of a reaction during which anintermittent detection method is active may be referred to as a“detection period” and a stage of a reaction during which anintermittent detection method is inactive may be referred to as a“non-detection period.” In illuminated reactions, such periods may alsobe referred to as “illuminated periods” and “non-illuminated periods,”respectively, although it is to be understood that the term“non-illuminated period” included periods in which illumination may bepresent but altered as compared to illumination during an “illuminatedperiod.” For example, a non-illuminated period may be characterized by acomplete absence of illumination, or a modification of illumination,including but not limited to changes in wavelength, frequency,intensity, and/or number of illumination sources. Alternatively oradditionally, reaction components that are excited by the illuminationsource(s) may be modified or removed from a reaction mixture to create anon-illuminated period. For example, a fluorescent dye detected duringan illuminated period may be removed from the reaction mixture, e.g., bybuffer exchange, thereby producing a non-illuminated period during whichtime the fluorescent dye cannot be detected even if the excitationillumination is present. In a further example, a non-illuminated periodcan indicate a period during an illuminated reaction during which a typeof illumination-based detection that occurs during an illuminated periodis not occurring, e.g., the identity of fluorescently labelednucleotides incorporated into a nascent strand is not being detected orrecorded.

In certain aspects, the present invention is generally directed toimproved methods, devices, and systems for performing illuminatedreactions. The term “illuminated reactions” as used herein refers toreactions which are exposed to an optical energy source. Typically, suchillumination is provided in order to observe the generation and/orconsumption of reactants or products that possess a particular opticalcharacteristic indicative of their presence, such as a shift in theabsorbance spectrum and/or emission spectrum of the reaction mixture orits components. In certain preferred embodiments, illuminated reactionscomprise one or more fluorogenic or fluorescent components. Inaccordance with certain methods of the invention, such illuminatedanalyses are subjected to intermittent detection (e.g., data collection)for one or more aspects of the data typically collected for a givenreaction. For example, aspects of the data typically collected fornucleotide sequencing reactions include nucleotide sequence data, readquality data, signal to background ratios, reaction rates and durations,measures of the fidelity of the reaction, reaction times, and the like.In certain preferred embodiments, nucleotide sequence data isiteratively collected during an ongoing sequencing reaction to generatenucleotide sequence reads for at least two or more noncontiguous regionsof a template nucleic acid molecule. Such iterative sequence dataacquisition may be achieved in various ways depending on the sequencingtechnology in use. For example, in sequencing methods that utilizeluminescent components that generate a signal indicative of the identityof a base position, iterative sequence data collection may be achievedby removing or altering an illumination source (or a reaction relativeto an illumination source), substituting the luminescent components forunlabeled components that do not generate signal, or otherwiseinterrupting signal acquisition in the experimental system.

In certain preferred embodiments, such illuminated reactions areilluminated for an amount of time that permits the effective performanceof the analysis. Traditionally, illuminated reactions are illuminatedfrom initiation through completion, and the time during which reactiondata may be reliably collected is dictated by the progression (asmeasured by, e.g., processivity, rate, fidelity, duration, etc.) of thereaction under constant illumination. Some reactions are sensitive tosuch constant illumination, which can reduce their performance (e.g.,processivity), and thereby prevent collection of data from later stagesof the reaction, i.e., stages that would otherwise occur if the reactionwere carried out with no illumination. The present invention providesmethods for performing illuminated reactions comprising subjecting thereactions to intermittent illumination. Such intermittent illuminationcan increase performance (e.g., processivity, rate, fidelity, duration,etc.) of the reactions, thereby allowing generation of data that cannotbe collected under constant illumination, such as data from later stagesof an ongoing reaction whose progression is compromised under constantillumination. For example, in sequencing-by-incorporation reactions theuse of intermittent excitation illumination can increase processivity,which has the benefit of providing sequence reads more distal from thepolymerase binding/initiation site than such reactions subjected toconstant exposure to excitation illumination.

Further, it is an object of the instant invention to provide sequencedata from noncontiguous regions of a nucleic acid template in a singlereaction. Other commercially available platforms have attempted toachieve such noncontiguous sequence data through, e.g., complex cloningand sequencing strategies. The present invention provides a clearadvantage over such strategies by providing a simple and economicalsolution that is applicable across various platforms, and isparticularly applicable to illuminated, single-moleculesequencing-by-incorporation reactions.

In preferred embodiments, illuminated reactions for use with the instantinvention are nucleic acid sequencing reactions, e.g.,sequencing-by-incorporation reactions. In preferred embodiments, such anilluminated reaction analyzes a single molecule to generate nucleotidesequence data pertaining to that single molecule. For example, a singlenucleic acid template may be subjected to a sequencing-by-incorporationreaction to generate one or more sequence reads corresponding to thenucleotide sequence of the nucleic acid template. For a detaileddiscussion of such single molecule sequencing, see, e.g., U.S. Pat. Nos.6,056,661, 6,917,726, 7,033,764, 7,052,847, 7,056,676, 7,170,050,7,361,466, 7,416,844; Published U.S. Patent Application Nos.2007-0134128 and 2003/0044781; and M. J. Levene, J. Korlach, S. W.Turner, M. Foquet, H. G. Craighead, W. W. Webb, SCIENCE 299:682-686,January 2003 Zero-Mode Waveguides for Single-Molecule Analysis at HighConcentrations, all of which are incorporated herein by reference intheir entireties for all purposes. In some embodiments, a plurality ofsingle nucleic acid templates are analyzed separately and oftensimultaneously to generate a plurality of sequence reads correspondingto the nucleotide sequences of the plurality of nucleic acid templates.In certain preferred embodiments, the plurality of nucleic acidtemplates includes at least two nucleic acid templates that compriseidentical nucleotide sequences such that analysis of the two nucleicacid templates generates overlapping sequence reads. In certainpreferred embodiments, at least one of the nucleic acid templates isconfigured to provide redundant sequence data in a single sequence read,e.g., via duplications, sense and antisense sequences, and/orcircularization.

Certain aspects of the invention are directed to methods, devices, andsystems for generating a sequence scaffold for a nucleic acid template,e.g., chromosome, genome, or portion thereof. A sequence scaffold asused herein refers to a set of sequence reads that extends across atleast a portion of a nucleic acid template. In some embodiments, such asequence scaffold is used to generate a consensus sequence for thenucleic acid template. In some embodiments, the nucleic acid template isvery large, e.g., at least about 100, 1000, 10,000, 100,000, or morebases or base pairs in length. In some embodiments, the sequencescaffold and/or consensus sequence is based on at least 1-, 2-, 5-, 10-,20-, 50-, 100-, 200-, 500-, or 1000-fold coverage of at least a portionof the nucleic acid template. In some preferred embodiments, the portionof the nucleic acid is at least about 10%, 20%, 30%, 40%, 50%, 60%, 70%,80%, 90%, or 100% of the entire length of the nucleic acid template.

In certain aspects, the invention is particularly suitable forsequencing nucleic acid templates interspersed with repetitive elements.Such repetitive elements present major logistical and computationaldifficulties for assembling fragments produced by sequencing strategies,especially those with read-lengths that are too short to encompassunique reads outside the repeat region. For example, the human T-cellreceptor locus contains a five-fold repeat of a trypsinogen gene that is4 kbp long and that varies 3 to 5% between copies. Therefore, asequencing strategy that cannot provide nucleotide sequence informationthat spans at least 20 kb for a single molecule containing the locuswill have difficulty providing consensus sequence for the locus.Further, Alu repeats (˜300 bp retrotransposons) are also problematicbecause they cluster and can constitute up to 50-60% of the templatesequence, with copies varying from 5-15% between each other. The humangenome contains an estimated one million Alu repeats and 200,000 LINEelements (average length ˜1000 bp), representing roughly 10% and 5% ofthe entire genome, respectively. In certain embodiments, the presentmethods facilitate efficient and accurate sequence determination forlong templates comprising such repetitive sequences, in part because thepresent methods do not rely solely on sequence overlap to generateconsensus sequences, but also include information related to theexpected location of the polymerase on the template nucleic acid,thereby linking a particular sequence read to a particular location onthe template nucleic acid. This greatly facilitates accurate assembly ofsequence reads to generate sequence scaffolds and/or consensussequences.

Certain aspects of the invention are directed to methods, devices, andsystems for generating multiple sequence reads in an illuminatedsequencing-by-incorporation reaction that are distal from one another(i.e., noncontiguous) on a single nucleic acid template by removing theexcitation illumination during the course of the reaction, andsubsequently reinitiating the excitation illumination. Sequence readsare generated only during the periods of time when the excitationillumination is present, resulting in a “gap” between the sequence readsfrom a single template nucleic acid that corresponds to the time duringwhich the excitation illumination was absent but the incorporation ofnascent nucleotides continued “in the dark.” As such, the number ofsequence reads generated for a given template nucleic acid is equal tothe number of periods during which the excitation illumination ispresent.

Certain aspects of the invention are directed to methods, devices, andsystems for generating multiple sequence reads from a plurality ofnucleic acid templates comprising identical nucleotide sequences. Insome embodiments, the multiple sequence reads are not all from the sameregion of the nucleic acid templates. In some embodiments, there isoverlap between the multiple sequence reads. In some embodiments, asingle sequence read is generated from each of the plurality of nucleicacid templates, and in other embodiments multiple noncontiguous sequencereads are generated from each of the plurality of nucleic acidtemplates. In certain preferred embodiments, the multiple noncontiguoussequence reads from each of the plurality of nucleic acid templatestogether extend across the nucleic acid templates such that they can becombined to provide a consensus sequence for the identical nucleotidesequence in the nucleic acid templates. In some embodiments, theconsensus sequence is based on at least 2-, 5-, 10-, 20-, 50-, 100-,200-, 500-, or 1000-fold coverage of the identical nucleotide sequence.In some embodiments, the identical nucleotide sequence represents atleast about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100% of thenucleic acid template.

Certain aspects of the invention are directed to methods, devices, andsystems for reducing or limiting the effects of photo-induced damageduring illuminated reactions, particularly reactions that employfluorescent or fluorogenic reactants. The term “photo-induced damage”refers generally to any direct or indirect impact of illumination on oneor more reagents in a reaction resulting in a negative impact upon thatreaction. Without being bound to a particular theory or mechanism ofoperation, some illuminated reactions are subject to photo-induceddamage that can hinder progression of the reaction, e.g., via damage toreaction components, such as enzymes, cofactors, templates, etc. Assuch, the illumination of the illuminated reaction can directly orindirectly negatively impact progression of the reaction, and such animpact can be measured based on various characteristics of the reactionprogression, e.g., processivity, rate, fidelity, duration, etc. Thepresent invention provides methods for subjecting an illuminatedreaction to intermittent exposure to illumination, which reduces theamount of photo-induced damage at a given time during the reaction,allowing the reaction to proceed further than it does when constantlyexposed to the illumination.

In some embodiments, the methods herein may further comprise theaddition of one or more photo-induced damage mitigating agents (e.g.,triplet-state quenchers and/or free radical quenchers) to theilluminated reaction. Such photo-damage mitigating agents are generallyknown to those of skill in the art. Further discussion of photo-induceddamage and related compounds, compositions, methods, devices, andsystems are also provided in U.S. Pub. No. 20070161017, filed Dec. 1,2006; and U.S. Ser. No. 61/116,048, filed Nov. 19, 2008, which areincorporated by reference herein in their entireties for all purposes.

II. Intermittent Illumination of Analytical Reactions

Certain aspects of the invention are generally directed to improvedmethods for performing illuminated analyses. The terms “illuminatedanalysis” and “illuminated reaction” are used interchangeably andgenerally refer to an analytical reaction that is occurring while beingilluminated (e.g., with excitation radiation), so as to evaluate theproduction, consumption, and/or conversion of luminescent (e.g.,fluorescent) reactants and/or products. As used herein, the terms“reactant” and “reagent” are used interchangeably. As used herein, theterms “excitation illumination” and “excitation radiation” are usedinterchangeably. In certain embodiments, the illuminated reaction is asequencing reaction, e.g., a sequencing-by-incorporation reaction. Incertain embodiments, the illuminated reaction is designed to analyze asingle molecule, e.g., by ensuring the molecule is optically resolvablefrom any other molecule being analyzed and/or in the reaction mixture.In certain embodiments, one or more components of the reaction aresusceptible to photo-induced damage directly or indirectly elicited byan excitation radiation source. In certain preferred embodiments, anilluminated reaction is subjected to intermittent excitation radiationduring the course of the illuminated reaction. In certain preferredembodiments, a sequencing-by-incorporation reaction is subjected tointermittent excitation radiation during the course of a polymerizationreaction to generate a plurality of noncontiguous sequence reads from asingle nucleic acid template.

In certain aspects, the methods herein provide benefits over methodscurrently used for sequencing large template nucleic acids, such ashuman genomes. For example, the traditional shotgun sequencing approachentails sequencing nucleic acid fragments and analyzing the resultingsequence information for overlap and similarity to known sequences toconstruct the complete sequence of the template nucleic acid. Onedisadvantage to the shotgun approach is that assembly may be difficultif the template nucleic acid comprises numerous repeated sequences, andthe inability to assemble a genomic sequence in repeat regions leads togaps in the assembled sequence. (See, e.g., Myers, G.; “Whole-Genome DNASequencing” in Computing in Science and Engineering; Vol 1, Issue 3;pgs. 33-43; May/June 1999.) One method of resolving these gaps is tosequence fragments large enough to span the repeat regions, butsequencing large fragments can be difficult and time-consuming. Anotherapproach to spanning a gap is to determine the sequence of two ends of alarge fragment which has known spacing and orientation, and thisapproach is generally termed paired end sequencing (see, e.g., Smith, M.W. et al., (1994) Nature Genetics 7:40-47; and U.S. Pub. No.2006/0292611, filed Jun. 6, 2006, both of which are incorporated byreference herein in their entireties for all purposes). This method islimited by the requirement for information about the spacing andorientation of the ends of the long fragment, and/or complex samplepreparation of the nucleic acid template. The present invention providesmethods that are tolerant of large repetitive regions and do not requireprior knowledge of nucleotide sequences (e.g., base sequences, spacing,orientation, etc.) or complex sample preparation, thereby allowingeconomical, efficient, and effective de novo sequencing or resequencingof long template nucleic acids.

In certain aspects, the methods herein provide various strategies forachieving intermittent illumination of illuminated reactions.Essentially, at least one type of illumination (e.g., excitationillumination) is present for at least one time period (“illuminatedperiod”) and absent during at least one other time period(“non-illuminated period”) during an illuminated reaction. As describedabove, the term “non-illuminated” indicates a change in illuminationincluding, but not limited to a complete absence of illumination. Forexample, a non-illuminated period may also be characterized by adifferent illumination source or intensity than an illuminated period,or by a change in reaction components, e.g., detectable labels. Ingeneral, at least one type of data collected during an illuminatedperiod (e.g., nucleotide sequence data) is not collected during anon-illuminated period. An absence of the illumination may be due to,e.g., inactivation of the illumination source (e.g., laser, laser diode,a light-emitting diode (LED), a ultra-violet light bulb, and/or a whitelight source), removal of the illuminated reaction from the illuminationsource (or vice versa), or may be due to blockage of the illuminationfrom the reaction, as discussed below. Modifications to the illuminationmay be due to, e.g., adjustment of the intensity of an illuminationsource, or a substitution of one illumination wavelength and/orfrequency for another. Further, components detectable during anilluminated period may be removed from the reaction mixture during anon-illuminated period, e.g., a fluorescently labeled nucleotide may bereplaced with an unlabeled nucleotide. Knowledge of the rate of thereaction and the time during which the illumination is absent is used toestimate the progress of the reaction during the non-illuminated period.For example, if a reaction proceeds such that one molecule isincorporated into a macromolecule per second, and the illumination isabsent for 20 seconds, it can be estimated that 20 molecules wereincorporated during the non-illuminated period. This information isuseful during data analysis to provide context for the reaction datacollected during the illuminated period(s). For example, in asequencing-by-incorporation reaction the number of base positionsseparating sequence reads generated in illuminated periods can beestimated based on the temporal length of intervening non-illuminatedperiods and the known rate of incorporation during the reaction and/orby the measured rate of incorporation during the illuminated period(s).The known rate of incorporation can be based on various factorsincluding, but not limited to, sequence context effects due to thenucleotide sequence of the template nucleic acid, kinetics of thepolymerase used, buffer effects (salt concentration, pH, etc.), and evendata being collected from an ongoing reaction. Further the processivityof an enzyme during a non-illuminated period (or other type ofnon-detection period) can be manipulated or adjusted by methods known tothose of skill in the art. In particular, the kinetics of replication bya polymerase enzyme can be altered by changing the chemical environmentin which it operates, and such methods are further described, e.g., inU.S. patent application Ser. No. 12/414,191, filed Mar. 30, 2009; Ser.No. 12/537,130, filed Aug. 6, 2009; and U.S. patent Application No.[unassigned], attorney docket no. 105-006301US, entitled “EngineeringPolymerases and Reaction Conditions for Modified IncorporationProperties,” filed Sep. 4, 2009, the disclosures of all of which areincorporated herein by reference in their entireties for all purposes.For example, methods are provided for adjusting the enzyme activity, andthese methods find particular relevance in the instant invention whenused to enhance accuracy during detection periods, and to enhanceprocessivity during non-detection periods. Information regarding enzymetranslocation rate and processivity is useful for positioning thesequence reads for a single template nucleic acid relative to oneanother in the construction of a sequence scaffold and/or consensussequence for the template nucleic acid.

FIG. 1 provides exemplary embodiments of methods for intermittentillumination of analytical reactions. A reaction mix is prepared at step100. In the process shown on the left, illumination of the reaction 105is begun prior to initiation of the reaction 110, which allows“illumination data” to be collected at initiation. (In an alternativeembodiment, illumination may commence simultaneously with initiation ofthe reaction.) “Illumination data” as used herein refers to datacollected during an illuminated period, e.g., the length of theilluminated period and luminescent signal(s) from the reaction product.At least one non-illuminated period 115 occurs during the course of thereaction, followed by at least one additional illuminated period 120.Multiple additional non-illuminated and illuminated periods may follow.During the illuminated periods (105 and 120), illumination data iscollected 175. During the non-illuminated period(s), non-illuminationdata is collected 180. As used herein, “non-illumination data” refers todata collected during a non-illuminated period, e.g., the length of thenon-illuminated period can be monitored. In the process shown on theright, the reaction is initiated 155 during a first non-illuminatedperiod 150. At least one illuminated period 160 occurs during the courseof the reaction, optionally followed by at least one additionalnon-illuminated period 165. Multiple additional illuminated andnon-illuminated periods may follow. As for process A, illumination datais collected 175 during the illuminated period(s) 160, andnon-illumination data is collected 180 during non-illuminated periods(155 and 165).

One benefit provided in certain embodiments of the invention is that thereaction need not be further manipulated after initiation (aside fromthe control of illumination). For example, the method can be used toanalyze reaction mixtures without the need for buffer changes, additionof further reaction components, or removal of detectable components,e.g., light-activatable components such as fluorophores. For example, ina sequencing-by-incorporation reaction, labeled nucleotides may bepresent throughout the life of the reaction, even when the reaction isnot generating nucleotide sequence data (e.g., during a non-illuminatedperiod). This provides clear advantages over methods that requireadditional handling of the reaction after initiation, which tend to notonly be expensive and time-consuming, but which also provideopportunities for contamination of the reaction. For example,illumination can be reinitiated at any time during the reaction at thewhim of the ordinary practitioner by simply activating the illumination.In certain preferred embodiments, the concentration of labelednucleotides or nucleotide analogs in the reaction mixture is greaterthan the concentration of unlabeled nucleotides in the reaction mixturethroughout the course of the reactions, and may represent at least about60%, 70%, 80%, 90%, 95%, 99%, or 100% of the total nucleotides in thereaction mixture. Methods for ensuring a high ratio of labeled versusunlabeled nucleotides in a reaction mixture are known in the art andcertain preferred embodiments are provided in U.S. Patent Pub. Nos.2006/0063264, 2006/0194232, and 2007/0141598, which are incorporatedherein by reference in their entireties for all purposes.

In embodiments in which a sequencing-by-incorporation reaction issubjected to intermittent illumination, the sequence reads collectedduring the illuminated periods are arranged in order and separated fromone another by an estimated number of nucleotides incorporated into thenascent strand during the intervening non-illuminated periods. Theresulting gapped read can then be used to assess certain characteristicsof the template nucleic acid. When multiple identical template nucleicacids are subjected to such a sequencing-by-incorporation reaction, theresulting set of gapped reads can be combined to create a sequencescaffold and/or a consensus sequence for the template nucleic acid.

Additional methods may also be used to aid in assembly of gapped readsinto a sequence scaffold and/or a consensus sequence for a templatenucleic acid. For example, in some embodiments, alternative labelingmethods can be used to provide additional data during the course of thereaction, e.g., data from illuminated or non-illuminated periods. Incertain preferred embodiments, such alternative labeling methods maycomprise using labels that are incorporated into a product of thereaction. For example, in sequencing-by-incorporation reactions that usenucleotides comprising labeled terminal phosphates (e.g., the gammaphosphate as in dNTP, or terminal phosphates on nucleotide analogs witha greater number of phosphate groups) to identify the nucleotidesincorporated into a nascent polynucleotide, the reaction mixture mayalso include nucleotides comprising a base-linked label. During thereaction, these “base-labeled nucleotides” will be incorporated into thenascent strand, but unlike the terminal phosphate labels removed duringincorporation, the base-linked labels are not cleaved from thenucleotide upon incorporation by the polymerase, resulting in a nascentstrand that comprises the base-linked labels. The concentration of suchbase-labeled nucleotides can be adjusted in the reaction mixture topromote their incorporation into the nascent strand at a predictablerate, e.g., based on the known sequence of the template or the averagefrequency of a given nucleotide. The presence and/or rate ofincorporation of the base-linked labels into the nascent strand canprovide a measure of the length of the nascent strand generated (and,therefore, the distance traveled by the polymerase along the templatenucleic acid) during the reaction by subjecting the reaction toexcitation illumination that excites the base-linked label (butpreferably not the non-base-linked labels), and detecting the signalemitted. The excitation of the base-linked labels preferably occurs as apulse during or immediately following a non-illuminated period, and isotherwise absent during the reaction. The strength of the signal isindicative of how many labels are present in the nascent strand, therebyproviding a measure of the processivity of the polymerase for a givenperiod during the ongoing reaction, e.g. during one or more illuminatedor non-illuminated periods. Since the base-linked labels remain in thenascent strand, it is beneficial to minimize the amount of time thosefluorophores are subjected to excitation illumination to mitigate thepotential of photo-induced damage to the reaction components. As such,in preferred embodiments, the excitation illumination wavelength for thebase-labeled nucleotides is different than that of other fluorescentlabels in the reaction.

This method can be modified in various ways. For example, thebase-labeled nucleotides may also comprise a terminal phosphate label sothat their incorporation can be monitored in the same manner during anilluminated period as the non-base-labeled nucleotides. There may be asingle type of base-labeled nucleotide in a reaction mixture, ormultiple types may be present, e.g., each type carrying a differentnucleobase. The concentration of base-labeled nucleotides in thereaction mix may be varied, although it is preferred that the ratio ofbase-labeled nucleotides to non-base-labeled nucleotides be relativelylow. For example, in a reaction mixture comprising a single type ofbase-labeled nucleotide (e.g., base-labeled dATP), it is preferred thatthe ratio of base-labeled dATP to non-base labeled dATP be less than1:8, and more preferably 1:10 or less. The low concentration ofbase-labeled nucleotides is preferred in order to minimize stericallyinduced polymerase stalling when incorporating multiple base-labelednucleotides in a row. In some embodiments, the optimal ratio ispre-determined using capillary electrophoresis for any specificbase-labeled nucleotide and likely homopolymer sequence prevalence. Incertain preferred embodiments, at least 50, 75, 100, 125, or 150base-labeled nucleotides are incorporated into the nascent strand duringa single non-detection period. The base-labeled nucleotides may bepresent throughout the reaction, or may be washed in duringnon-illuminated periods and washed out after the pulse of excitationillumination. The reaction mixture comprising base-linked nucleotidesbeing washed in may also include unlabeled nucleotides for incorporationduring a non-detection period. During a subsequent illuminated period, areaction mixture comprising terminal phosphate-labeled nucleotidesreplaces the reaction mixture comprising base-linked nucleotides andunlabeled nucleotides. This protocol is one embodiment of the methods ofthe invention in which a non-detection period is not necessarily anon-illuminated period because in this case illumination may be present,but no incorporation of nucleotides is be detected.

Alternatively or in addition, a low concentration of a fifth terminalphosphate labeled nucleotide can be present in the sequencing reaction,wherein the label has a different excitation wavelength than the otherlabels in the reaction mixture. For example, a small proportion of onenucleotide analog, e.g., dA6P, can be labeled with the “fifth label.”During non-detection periods when the sequence of incorporation ofnucleotides is not being monitored, the reaction site is illuminated byexcitation radiation specific for the fifth label, and this fifth labelexcitation radiation can be inactivated during the detection periods.Emissions detected upon incorporation of the nucleotide analogcomprising the fifth label are used to “clock” the pace of thepolymerase during the non-detection period, e.g., based upon the knownor estimated frequency of the complementary nucleotide in the templatestrand. The fifth label can be chosen such that the excitation andemission radiation are less likely or unlikely to cause photo-induceddamage to reaction components, e.g. by choosing a label with a longexcitation wavelength (e.g., toward the red end of the visiblespectrum), a label that has a low propensity for entering into a tripletstate, and/or a label that has a low propensity to form a radical. Sincethe fifth label is being excited when other labels are not, there is norequirement for optimal spectral separation from other labels in thereaction mixture. Further, since the fifth label is not being used forsequencing, other optimizations are also not necessary, e.g., related tobranching, accuracy, and the like. Various types of labels can be usedas a fifth label of the invention including, but not limited to, organicand non-organic dye fluorophores. For example, latex nanoparticles orquantum dots are particularly suitable due to their lower propensity forphoto-induced damage of certain analytical reaction components. Incertain preferred embodiments, a quantum dot label has an emissionspectrum within the same spectral window as the labels that are used toidentify the sequence of base incorporations into the nascent strand(“sequencing labels”) but an excitation spectrum that does not overlapthose of the sequencing labels to allow detection of the fifth labelemissions using the same optical system as is used to detect thesequencing label emissions.

This method can be modified in various ways. For example, more than onesmall subset of a nucleotide analog can be labeled with a fifth label,and in certain embodiments, a small subset of each nucleotide analogpresent in the reaction mixture is labeled with the fifth label.Further, there may be a plurality of additional labels present in thereaction, each of which is present on a small subset of a single type ofnucleotide analog, e.g., sixth, seventh, and eighth labels. Byincreasing the number of types of nucleotide analogs labeled with fifth(or sixth, seventh, eighth) labels, their frequency of incorporation islikewise increased, which improves the translocation rate calculationfor the polymerase during the non-detection periods. Alternatively, eachtype of nucleotide analog can comprise both a sequencing label that isspecific for the cognate base in the nucleotide, as well as a fifthlabel for clocking the polymerase. The sequencing labels are excited anddetected during the detection periods and the fifth labels are excitedand detected during the non-detection periods. Since every nucleotideanalog is labeled with a fifth base, each incorporation event can becounted during the non-detection period and the exact rate ofincorporation can be determined. Both the sequencing and fifth labelsmay be bound to the same or different linkers on the nucleotide analogs.In certain preferred embodiments, a linker on a nucleotide analogpositions the fifth label within an illumination zone to allowexcitation, but far from an enzyme (e.g., polymerase) to mitigatephoto-induced damage related to excitation of and/or emission from thefifth label.

In some embodiments, the fifth label is also excited by an illuminationduring the detection periods. The availability of the clocking functionduring the detection period can be used during sequence analysis toidentify positions in the resulting sequence read where a signal was notdetected (resulting in an apparent “missing base” in the read) and todistinguish between true insertions and branching events in which twosignals are detected for a single incorporation event.

In yet further embodiments, assembly of gapped reads into a sequencescaffold and/or a consensus sequence for a template nucleic acid isfacilitated by using “non-illuminated periods” characterized by modifiedexcitation illumination rather than a complete absence of excitationillumination (which can also be termed “low-illuminated periods”). Forexample, in some embodiments a lower intensity excitation illuminationis used during the non-illuminated periods that excites one or more ofthe labels that are excited during the illuminated periods. As such,unlike various strategies described above, no fifth label is necessary.The lower intensity excitation illumination results in emissions thatare lower intensity but still intense enough to identify an emissionsignal over background counts, though typically not intense enough to beused to identify the particular label generating the emission signal.For example, if label “A” and label “B” are in a reaction mixture,during an illuminated period the intensity of the signal emissions fromeach are high enough that the artisan can distinguish from which label aparticular signal originates by the wavelength and/or frequency of thesignal. However, during a low-illuminated period the artisan can onlyidentify that a signal emission occurs, but is unable to distinguish theoriginating label because its particular wavelength and/or frequencycannot be accurately determined. The decrease in excitation illuminationintensity provides both a mitigation of photo-induced damage to reactioncomponents within the observation volume while allowing the practitionerto count the emissions, and therefore the incorporations, during thenon-illuminated period.

In other embodiments, multiple excitation illumination sources are usedduring an illuminated period, and a first subset of these illuminationsources is removed during a non-illuminated period, while a secondsubset remains. The illumination sources that remain during thenon-illuminated period may be present in the same manner as during theilluminated period, or various aspects may be altered, e.g., intensitymay be reduced. For example, if labels A and B present in a reactionmixture are excited by a first illumination source and labels C and Dpresent in the reaction mixture are excited by a second illuminationsource, removal of the first illumination source during thenon-illuminated period results in an inability to detect labels A and B,while C and D are still detectable. Such an incomplete data set can beused to clock the progress of the reaction during the non-illuminatedperiod(s). Further, it can also be used in various ways to facilitatethe statistical analysis of data collected during the illuminatedperiod(s). For example, for nucleotide sequencing applications (asdescribed elsewhere herein) the incomplete data set(s) collected duringnon-illuminated period(s) can be used during assembly of a sequencescaffold. For example, during de novo sequence assembly a collection ofsequences (contigs) are generated, but the order of the contigs relativeto the template nucleic acid is not always apparent. The scaffoldingprocess uses extra information to determine the correct order of thecontigs. So, if only two bases are identifiable in the non-illuminatedperiods, the incomplete sequence reads comprising only incorporation ofthese two bases can be aligned to modified versions of the contigsassembled from data collected during an illuminated period, but in whichthe two bases not detected during the non-illuminated periods have beenremoved. Once the order of the contigs has been determined, theincorporation data for the two bases not detected during thenon-illuminated periods is restored and the assembly of the contigs iscomplete. This method can be modified in various ways. For example, thepractitioner may choose which illumination sources to remove during thenon-illuminated periods based on various characteristics, such as theirpropensity to cause photo-induced damage to one or more reactioncomponents, the propensity of the corresponding emission signal to causephoto-induced damage to one or more reaction components; their energyconsumption; and wear-and-tear on the source device. Further, asdescribed elsewhere herein, rather then removing an illumination source,reaction components that are excited by the illumination source may beremoved from the reaction mixture during the non-illuminated period,necessarily rendering them undetectable. For example, one or morefluorescently labeled nucleotide analogs may be replaced with unlabelednucleotide analogs during the non-illuminated periods.

In certain aspects, the invention provides advantages to performingintramolecular redundant sequencing, in which a template nucleic acid isused to generate multiple copies of a sequence read of interest, whetherby virtue of multiple copies of the complement being present in thetemplate, repeated replication of the template, or a combinationthereof. For example, a first stage of a template-dependent sequencingreaction on a single-stranded circular template can comprise anon-illuminated period during which the template is completelyreplicated at least one time to generate at least one incompletesequence read for a sequence complementary to the template. The firststage is followed by a second stage comprising an illuminated periodduring which the template is replicated multiple times to generatemultiple complete sequence reads for the complementary sequence. Theincomplete reads generated in the first stage can be used to construct ascaffold for assembly of the complete sequence reads generated in thesecond stage. Further, incomplete sequence reads can also be used toclock the progress of the reaction during the non-illuminated periods byproviding a count of the detectable reaction components and combiningthat information with known or estimated characteristics of thetemplate, e.g., nucleotide composition or sequence.

The subset of signal emissions detectable in the non-illuminated periodsas compared to the number detectable in the illuminated periods is notlimiting and may be chosen based upon the non-illumination data desiredby the ordinary practitioner and/or other considerations, such asmitigation of photo-induced damage to extend readlength. For example, tolower the likelihood of photo-induced damage, the ordinary practitionermay choose to remove the illumination source that is most damaging,e.g., has the highest frequency. In certain embodiments, multiplesequencing reactions may be performed for a single amplified template,each with a different combination of illumination sources and/ordetectable components. Alternatively or additionally, multiple replicatereactions can also be performed for one or more of the combinations ofillumination sources and/or detectable components. The combination ofdata from multiple different and/or replicate reactions performed on asingle template provides myriad benefits during statistical analysis. Asnoted above, data can be combined to facilitate assembly of contigsgenerated during illuminated periods. Data from non-illuminated periodscan also provide value in assessing the quality of the sequence readsgenerated during the illuminated periods.

Additional methods may also be used to aid in assembly of gapped readsinto a sequence scaffold and/or a consensus sequence for a templatenucleic acid. For example, in some embodiments, alternative labelingmethods used to provide additional data during the course of thereaction can comprise using labels that are incorporated into an enzymeof the reaction. For example, FRET labels can be used to label portionsof a polymerase enzyme such that the conformational change between theopen and closed states of the enzyme change the FRET value. For example,a FRET-based system can be used to monitor the kinetics of opening andclosing of the finger subdomain of DNA polymerase, as described inAllen, et al. (2008) Protein Science 17:401-408, incorporated herein byreference in its entirety for all purposes. In certain preferredembodiments, a closed conformation produces a FRET signal because thedonor and acceptor are close to one another, and an open conformationsilences the signal because there is no energy transferred between thedonor and acceptor. By monitoring the emission from the FRET pair, eachincorporation event can be monitored during non-detection periods, andoptionally or additionally during detection periods. In certainpreferred embodiments, the FRET donor is GFP (excitation at 484 nm;emission at 510 nm), and the FRET acceptor is YFP (excitation at 512 nm;emission at 529 nm). Methods for monitoring polymerase activity usingFRET labels are known in the art, e.g., in WO/2007/070572 A2, thedisclosure of which is incorporated herein by reference in its entiretyfor all purposes.

A given reaction may experience one or a plurality of illuminatedperiods or non-illuminated periods, but preferably experiences at leasttwo illuminated periods. For example, a given reaction providingnucleotide sequence information from a single template nucleic acid mayhave at least about 2, 3, 5, 10, 20, 50, or 100 illuminated periods withintervening non-illuminated periods. In an embodiment employing multipleperiods of illumination and/or non-illuminated, the periods may be thesame for both, e.g., 100 seconds “on” and 100 seconds “off.”Alternatively, the illuminated periods may be longer or shorter than thenon-illuminated periods. For example, in certain embodiments, anon-illuminated period may be at least about 2-, 3-, 4-, 6-, 8-, 10-,20-, or 50-fold longer than an adjacent illuminated period; or anilluminated period may be at least about 2-, 3-, 4-, 6-, 8-, 10-, 20-,or 50-fold longer than an adjacent non-illuminated period. Further, eachilluminated period may be the same or different from each otherilluminated period, and each non-illuminated period may be the same ordifferent from each other non-illuminated period. For example, someembodiments generate a smaller number of long reads, and otherembodiments generate a larger number of short reads. It will beunderstood that the number and length of the illuminated andnon-illuminated periods is limited only by the experimental system inuse and the data acquisition goals of the ordinary practitioner. In someembodiments, a nucleotide sequence read generated during a singleilluminated period comprises at least about 20, 30, 40, 50, 75, 100,1000, 10,000, 25,000, 50,000, or 100,000 adjacent nucleotide positions.In some embodiments, a region of a nucleic acid template processedduring a non-illuminated period during a single reaction comprises atleast about 20, 30, 40, 50, 75, 100, 1000, 10,000, 25,000, 50,000, or100,000 adjacent nucleotide positions. In some embodiments, the set ofnucleotide sequence reads generated during a single sequencing reactioncomprising a plurality of illuminated periods comprises at least about40, 60, 80, 100, 1000, 10,000, 25,000, 50,000, 100,000, 250,000,500,000, or 1,000,000 nucleotide sequence positions from a singlenucleic acid template. In some embodiments, a set of nucleotide sequencereads generated during a single sequencing reaction comprising aplurality of illuminated periods comprises multiple reads of at least aportion of the nucleotide sequence positions from a single nucleic acidtemplate.

As noted above, the present invention provides methods that are tolerantof large repetitive regions and do not require prior knowledge ofnucleotide sequences (e.g., base sequences, spacing, orientation, etc.).However, such information, if available, may also be useful to theordinary practitioner in determining an optimal periodicity forilluminated and non-illuminated periods during a sequencing reaction,especially when sequencing repetitive sequences. For example, if agenomic region is known to contain five adjacent copies of a onekilobase nucleotide sequence (i.e., five “repeat regions”), it would bebeneficial to keep the non-illuminated periods short enough to be ableto confidently map the resulting sequence reads to the correct repeatregion. If a non-illuminated period were too long, the natural variationin translocation rate of the polymerase would make it difficult toassign a sequence read to a particular repeat region, especially thosefarther from the binding/initiation site of the polymerase. In a furtherexample, if the “copies” each had a few mutations that could be used todistinguish them from each other, it would be beneficial to keep theilluminated periods long enough to increase the chance one of thesemutations would be included in a resulting sequence read, therebyallowing the unambiguous assignment of the read to a particular repeatregion. If the illuminated period were too short the sequence reads fromtwo different repeat regions could be identical, making mapping thesequence read challenging. (Another way to mitigate these difficultieswould be to incorporate pause or stop points into the template nucleicacid, as discussed below.)

Essentially, the practitioner may design the number of and lengths oftime for each illuminated and non-illuminated period to best suit theilluminated reactions being analyzed and the invention is not limited inthis regard. In certain embodiments, a practitioner may wish to increasethe processivity of a polymerase thereby extending the length of thetemplate nucleic acid processed in a sequencing reaction to be, e.g., atleast 2-, 3-, 4-, 6-, 8-, 10-, or 20-fold, thereby generating sequencedata much farther away from the polymerase binding/initiation site thanwould be achieved under constant illumination. In certain embodiments, apractitioner of the instant invention may wish to focus on data from oneor more stages of an ongoing reaction, such as stages for which moredata is required for analysis. In the case of sequencing-by-synthesis,one or more particular regions of a template nucleic acid may need to beresequenced. Some traditional methods require that new template nucleicacids be prepared to bring a region requiring resequencing closer to theinitiation point of the sequencing reaction, or require preparation ofmultiple new templates if multiple regions to be resequenced. Incontrast, the methods herein allow the practitioner to subject atemplate identical to the previously sequenced template (e.g., from alarge genomic DNA sample preparation) to a sequencing reaction whereinilluminated periods are timed to illuminate the sample only when thepolymerase is incorporating nucleotides into the nascent strand at theone or more particular regions requiring resequencing. This advantagesubstantially lowers the time and resources required for suchresequencing operations, therefore providing a significant advantageover traditional methods.

The instant invention contemplates various means for providingnon-illuminated periods during illuminated reactions. In someembodiments, the illumination source is turned off during the ongoingreaction to create one or more non-illuminated periods. In someembodiments, the illumination source remains on during the course of thereaction, but the illuminated reaction is removed from the system for aperiod of time. In some embodiments, the illumination source remains onduring the course of the reaction, but the illumination is blocked tocreate one or more non-illuminated periods. For example, a movable maskmay be manually or mechanically positioned between the illuminationsource and the illuminated reaction to block the illumination duringnon-illuminated periods and removed to allow exposure to theillumination during illuminated periods. Such a mask may also bedynamically controlled, such as a thin film transistor display (e.g., anLCD mask). Masks for blocking illumination and manufacture thereof arewell known to those of ordinary skill in the art and need no furtherelaboration herein.

One aspect of the present invention is multiplexing of large numbers ofsingle-molecule analyses. For a number of approaches, e.g., singlemolecule methods as described above, it may be desirable to provide thereaction components in individually optically resolvable configurations,such that a single reaction component or complex can be individuallymonitored. Providing such individually resolvable configurations can beaccomplished through a number of mechanisms. For example, by providing adilute solution of complexes on a substrate surface suited forimmobilization, one will be able to provide individually opticallyresolvable complexes. (See, e.g., European Patent No. 1105529 toBalasubramanian, et al., the full disclosure of which is incorporatedherein by reference in its entirety for all purposes.) Alternatively,one may provide a low density activated surface to which complexes arecoupled. (See, e.g., Published International Patent Application No. WO2007/041394, the full disclosure of which is incorporated herein byreference in its entirety for all purposes). Such individual complexesmay be provided on planar substrates or otherwise incorporated intoother structures, e.g., zero-mode waveguides or waveguide arrays, tofacilitate their observation.

In some embodiments, a plurality of illuminated reactions are carriedout simultaneously, e.g., on a solid support. In some preferredembodiments, a solid support comprises an array of reaction sites. Inpreferred embodiments, the reaction sites on a solid support areoptically resolvable from each other. In further preferred embodiments,each of the reaction sites on a solid support contains no more than asingle reaction to be interrogated. For example, in asequencing-by-incorporation embodiment, each reaction site preferablyhas no more than one polymerase and no more than one nucleic acidtemplate. The reaction sites may be confinements (e.g., optical and/orphysical confinements), each with an effective observation volume thatpermits resolution of individual molecules present at a concentrationthat is higher than one nanomolar, or higher than 100 nanomolar, or onthe order of micromolar range. In certain preferred embodiments, each ofthe individual confinements yields an effective observation volume thatpermits resolution of individual molecules present at a physiologicallyrelevant concentration, e.g., at a concentration higher than about 1micromolar, or higher than 50 micromolar range or even higher than 100micromolar. In addition, for purposes of discussion herein, whether aparticular reagent is confined by virtue of structural barriers to itsfree movement, or is chemically tethered or immobilized to a surface ofa substrate, it will be described as being “confined.”

As used herein, a solid support may comprise any of a variety offormats, from planar substrates, e.g., glass slides or planar surfaceswithin a larger structure, e.g., a multi-well plates such as 96 well,384 well and 1536 well plates or regularly spaced micro- or nano-poroussubstrates, or such substrates may comprise more irregular porousmaterials, such as membranes, aerogels, fibrous mats, or the like, orthey may comprise particulate substrates, e.g., beads, spheres, metal orsemiconductor nanoparticles, or the like. The solid support may comprisean array of one or more zero-mode waveguides or other nanoscale opticalstructures.

As used herein, “zero-mode waveguide” refers to an optical guide inwhich the majority of incident radiation is attenuated, preferably morethan 80%, more preferably more than 90%, even more preferably more than99% of the incident radiation is attenuated. As such high level ofattenuation, no significant propagating modes of electromagneticradiation exist in the guide. Consequently, the rapid decay of incidentelectromagnetic radiation at the entrance of such guide provides anextremely small observation volume effective to detect single molecules,even when they are present at a concentration as high as in themicromolar range. The fabrication and application of ZMWs in biochemicalanalysis, and methods for calling bases in sequencing-by-incorporationmethods are described, e.g., in U.S. Pat. Nos. 7,315,019, 6,917,726,7,013,054, 7,181,122, and 7,292,742, U.S. Patent Pub. No. 2003/0174992,and U.S. patent application Ser. No. 12/134,186, the full disclosures ofwhich are incorporated herein by reference in their entirety for allpurposes.

A set of reactions (e.g., contained on a solid support) may compriseidentical or different components. For example, a single templatenucleic acid may be analyzed in all reactions in the set, or a pluralityof template nucleic acids may be analyzed, each present in only one or asubset of the set of reactions. In preferred embodiments, templatenucleic acids comprising the same nucleotide sequence are analyzed in aplurality of reactions sufficient to provide adequate redundantnucleotide sequence data to determine a consensus sequence for thetemplate nucleic acids. A number of sequence reads that will provideadequate nucleotide sequence data will vary, depending, e.g., on thequality of the template nucleic acid and other components of thereaction, but in general coverage for a template nucleic acid orportion(s) thereof is at least about 2-, 5-, 10-, 20-, 50-, 100-, 200-,500-, or 1000-fold coverage. Further, the numbers and lengths ofilluminated and non-illuminated periods for a given reaction in the setof reactions may be the same or different than those for other reactionsin the set. In some embodiments, a mixture of different periodicitiesare used for a set of reactions comprising the same template nucleicacid. This strategy can be beneficial for providing nucleotide sequencereads from varying regions of the template sequence, thereby increasingthe likelihood of overlapping sequence reads between individualreactions. These overlapping sequence reads can facilitate constructionof a more robust sequence scaffold than could be constructed were thereactions all subjected to the same periodicity of illuminated andnon-illuminated periods.

Methods of controlling polymerase progress and/or synchronizingpolymerases in different reactions are also useful in analysis (e.g.,mapping, validation, etc.) of nucleic acid reads farther from theinitial binding site of the polymerase. During detection periods earlierin the reaction (i.e., closer to the time at which the polymerase beganto process the template nucleic acid, such as during a first illuminatedperiod), the position of a polymerase on the template can be estimatedwith generally good accuracy based on the known translocation rate ofthe polymerase under a given set of reaction conditions. As the durationof the reaction increases, however, the natural variation in polymerasetranslocation rate makes it more difficult to accurately determine theexact position of the polymerase on a template using estimation based ontranslocation rate alone; and through each subsequent illuminated periodsuch estimations of polymerase position become less accurate, makingsubsequent analysis and mapping of the sequence reads to the templatemore difficult. Methods of regulating the position of the polymerase onthe template allow more accurate determinations the polymerase'sposition. For example, causing the polymerase to pause or stop at agiven location on the template during a non-illuminated period andreinitiating the polymerization during or immediately prior to asubsequent illuminated period provides a way to reorient thesubsequently generated read with the template sequence, allowing easierconsensus sequence determination and mapping analyses. Further, suchpause/stop points can provide a means of controlling what regions of thetemplate are processed during the illuminated periods by restrictingwhere the polymerase will reinitiate on the template, thereby allowing apractioner of the instant invention to target one or more particularregions of a template for analysis during one or more detection periodsduring the course of an analytical reaction. Such methods are alsouseful to synchronize a set reactions being monitored simultaneously.For example, a plurality of reactions, each comprising a singlepolymerase/template complex, may be synchronized by regulating theinitiation points of the polymerase on the template for each detectionperiod, thereby creating a set of sequence reads that show lessspreading (i.e., less variation in the position on the template fromwhich the sequence reads are generated) in the later stages of thereactions than would otherwise be observed without such regulation.

Various methods can be used to control or monitor the progress of apolymerase on a template nucleic acid. For example, as noted above, onemay employ a reaction stop or pause point within the template sequence,such as a reversibly bound blocking group at one location on thetemplate, e.g., on the single-stranded portion that was not used inpriming. Reaction stop or pause points can be engineered into a portionof the template for which the nucleotide sequence is unknown (e.g., agenomic fragment), but is preferably located within a portion for whichthe nucleotide sequence is known (e.g., an adaptor or linker ligated tothe genomic fragment.) For example, certain preferred sequencingtemplates (e.g., SMRTbell™ templates, described elsewhere herein) areclosed, single-stranded molecules having regions of internalcomplementarity separated by hairpin or stem-loop linkers, and one orboth of these linkers can comprise a stop or pause point to control thepassage of the polymerase through them. In some embodiments, theseregulatory sequences or sites cause a permanent cessation of nascentstrand synthesis, and in other embodiments the reaction can bereinitiated, e.g., by removing a blocking moiety or adding a missingreaction component. Various types of pause and stop points are describedbelow and elsewhere herein, and it will be understood that these can beused independently or in combination, e.g., in the same templatemolecule.

By way of example, at a selected time following initiation ofpolymerization the reaction may be subjected to a non-illuminatedperiod. The incorporation of a synthesis blocking moiety coupled to thetemplate nucleic acid at a position encountered by the polymerase duringthe non-illuminated period will cause the polymerase to pause. Anexample of an engineered pause point is a known sequence on the templatenucleic acid where a primer sits and blocks progression of a polymerasethat is actively synthesizing a complementary strand. The presence ofthe primer by itself could introduce a pause in the polymerasesequencing or the primer could be chemically modified to force a fullstop (and synchronization of multiple polymerases in multiplereactions). The chemical modification could be subsequently removed (forexample, photo-chemically) and the polymerase would subsequentlycontinue along the template nucleic acid. In some embodiments, multipleprimers could be included in a reaction to introduce multiple pause orstop points along the template nucleic acid. Other methods for inducinga reversible pause (stop) in synthesis are known in the art and include,e.g., reversible sequestering of required cofactors (e.g., Mn²⁺, one ormore nucleotides, etc.). Once sufficient time has passed that thepolymerase is paused at the blocking group, illumination is reintroducedand the blocking group removed. This allows control of the position onthe template nucleic acid at which the polymerase will begin generatingnucleotide sequence data during the illuminated period. A variety ofsynthesis controlling groups may be employed, including, e.g., largephotolabile groups coupled to the template nucleic acid that inhibitpolymerase mediated replication, strand-binding moieties that preventprocessive synthesis, non-native nucleotides included within the primerand/or the template, and the like. Such reaction stops/pause points areuseful in providing more certainty about the relationship of the readsto each other. For example, since the exact position on a templatenucleic acid at which each sequence read begins would be known, theresulting reads could be better mapped relative to one another forconstruction of a sequence scaffold and/or consensus sequence. Furtherdescription of these and other methods for regulating the progress of apolymerase on a template are provided, e.g., in U.S. Ser. No.61/099,696, U.S. Patent Pub. No. 2006/0160113, and U.S. Patent Pub. No.2008/0009007, all of which are incorporated by reference herein in theirentireties for all purposes.)

By way of example, a sequencing reaction may be initiated on a templatecomprising a non-native base in the absence of the complement to thenon-native base, which would not impact the overall sequencedetermination of other portions of the template that are complementaryto native bases. By starving the reaction for the complement to thenon-native base, one can prohibit synthesis, and thus, the sequencingprocess, until the non-native base complement is added to the mixture.This can provide a “hot start” capability for the system and/or aninternal check on the sequencing process and progress that isconfigurable to not interfere with sequence analysis of the regions ofinterest in the template, which would be complementary to only nativebases. In some embodiments, the non-native base complement in thesequence mixture is provided with a detectably different label than thecomplements to the four native bases in the sequence, and the productionof incorporation-based signals associated with such labels provides anindication that the polymerase has initiated or reinitiated. Althoughdescribed as the “non-native base” it will be appreciated that this maycomprise a set of non-natural bases that can provide multiple controlelements within the template structure. In certain embodiments, twodifferent non-native bases are included within the template structure,but at different points, to regulate procession of the sequencingprocess, e.g., allowing controlled initiation and a controlledstop/start position later in the sequence, e.g., prior to a subsequentilluminated period. For example, the complement to the first non-nativebase can be added to initiate sequencing immediately prior to the startof a first illuminated period. During a first non-illuminated periodfollowing the first illuminated period, the polymerase encounters thesecond non-native base, e.g., at a nucleotide position near but upstreamof a nucleotide region desired to be sequenced in a second illuminatedperiod. Sequencing would stop until the complement to the secondnon-native base is added to the reaction mixture. Likewise, multiplesuch non-native bases could be incorporated into the template toeffectively target the polymerase to multiple regions of interest forwhich sequence data is desired. Further, in applications in whichmultiple identical templates are being sequenced, this would allow aresynchronization of the various sequencing reactions and the datagenerated therefrom.

Methods of controlling polymerase progress in different stages of asequencing reaction are also useful for not only creating“condition-dependent” non-detection periods (during which timeillumination may or may not be present), but also for minimizing theamount of time required for traversing a given length of template duringa non-detection period (whether or not illumination is present). Inorder to reliably detect incorporation events, non-natural reagentconditions are typically used to limit polymerization during detectionperiods to approximately 1-5, or about 3 bases per second. In certainembodiments, replacement of Mg²⁺ ions with Mn²⁺ ions serves to stabilizeand slow the translocation of the polymerase. When magnesium and,optionally, native nucleotides (e.g., lacking fluorescent labels) areused, the rate of translocation and/or processivity of the polymerasemay increase up to two orders of magnitude. Use of such “rapidtranslocation” conditions during the non-detection periods can providemyriad benefits, including but not limited to a more rapidpolymerization rate, an increased processivity (e.g., due to decreasedstalling and misincorporation), and an overall savings due to reduceduse of expensive labeled nucleotide analogs and/or reagents thatmitigate oxidative stress.

In certain embodiments, a protocol for intermittent detection comprisesalternating reaction mixtures, where a first reaction mixture usedduring the detection periods is optimized for sequence read generation,and a second reaction mixture used during the non-detection periods isoptimized for processivity and/or rapid polymerization. For example,when reagents for optimal sequence read generation are present, DNAsynthesis rate is low, and there is a fluorescence signal associatedwith each incorporation event. After replacing the reaction mixtureoptimized for sequence read generation with the reaction mixtureoptimized for processivity and/or rapid polymerization, the polymeraserapidly advances across the template. In certain embodiments, a flowcell is used to deliver and switch between the two (or more) reactionmixtures during the course of the reaction.

In an exemplary embodiment, a first reaction mixture comprisesfluorescently-labeled nucleotide analogs and manganese ions thatrestrict polymerization to a rate appropriate for high fidelitydetection of nucleotide incorporation. The first reaction mixture canalso include additional agents for mitigation of photo-induced damage ofvarious components of the reaction mixture. A second reaction mixturecomprises natural nucleotides and an appropriate magnesium ionconcentration for rapid synthesis of the nascent strand complementary tothe template. A first detection period of a sequencing reaction isinitiated by introduction of the first reaction mixture, and a sequenceread is generated based upon synthesis of the nascent strand during thedetection period. After a predetermined time interval a sufficientquantity of the second reaction mixture is flowed onto the reactionsite(s) until effectively all the first reaction mixture has beenreplaced with the second, thereby initiating a first non-detectionperiod. As noted above, the lack of labeled nucleotides in the secondreaction mixture alone can produce the non-detection period, since therewill be no signal emitted coincident with incorporation of the nativenucleotides, but in certain embodiments illumination may also beremoved, e.g., to further mitigate photo-induced damage during thenon-detection period. At a time appropriate to initiate a seconddetection period, a sufficient quantity of the first reaction mixture isflowed onto the reaction site(s) until effectively all the firstreaction mixture has been replaced with the second, and detection ofincorporation event is reinitiated. The cycle of reaction mixtureexchange is repeated to generate multiple detection and non-detectionperiods.

A flow cell for reaction mixture exchange preferably has two inputs thatare gated such that only a single reaction mixture flows into a reactionsite or plurality of reaction sites, e.g., on a substrate. A singleout-flow line may be used to remove reaction mixtures from the reactionsite(s) to a single collection vessel, or multiple collection vesselsmay be used, one for each type of reaction mixture used. Further,accurate estimation of the distance a polymerase translocates during anon-detection period is important for bioinformatics applications. Thisestimation is complicated if the time for reaction mixture exchange isslow. As such, the flow is preferably at a sufficient rate that the timefor exchange is significantly less than the time spent in the presenceof either reaction mixture alone.

FIG. 2 provides an exemplary embodiment of analysis of a plurality ofilluminated reactions using intermittent illumination. In thisembodiment, sixteen sequencing-by-incorporation reactions are performedon single nucleic acid templates (each of which comprises the samenucleotide sequence) with the timing of the illuminated andnon-illuminated periods the same for all sixteen reactions. In A, thesixteen reactions are shown disposed on sixteen reaction sites on asolid support and are numbered for convenience. A representation of theillumination data is shown in B, with bars extending across the graphindicative of illumination data collected during illuminated periods foreach reaction. In this illustrative example, each reaction is subjectedto three illuminated periods, each followed by a non-illuminated period,resulting in three noncontiguous sequence reads for each reaction, i.e.,three noncontiguous reads per template molecule sequenced. The positionof the bars relative to the x-axis provides the position of the sequenceread relative to the template nucleic acid sequence, which extends fromposition 0 (initiation of sequencing reaction) to n. During the firstilluminated period, the sequence reads generally overlap, but thenatural variation of polymerase translocation rate over the set ofreactions results in a “spreading” of the sequence reads as the reactionproceeds through the second and third illuminated periods withincreasing variation in the exact position of each polymerase on thetemplate at the beginning and end of each illuminated period. As such,the earlier illumination data provides better redundancy(“oversampling”) of sequence information over a relatively narrowportion of the template nucleic acid, while the later illuminatedperiods provide less redundant sequencing data over a broader region ofthe template nucleic acid. The timing of the non-illuminated periodsbetween the illuminated periods and the known or calculated rate ofincorporation are used to determine approximate spacing between theresulting sequence reads, providing context for building a sequencescaffold or consensus sequence. It is important to note that althoughshown disposed on a solid support in A, the data shown in B could alsohave been generated from reactions not disposed on a solid support norperformed simultaneously and the methods are generally not so limited.Further, as described above, the spreading of the sequence reads fromlater stages of the reactions can be mitigated by synchronizing thereactions, e.g., by regulating the initiation points of the polymeraseon the template for each detection period, thereby creating a set ofsequence reads that provides better redundancy (i.e., more overlap inthe positions on the template from which the sequence reads aregenerated), especially in the later stages of the reactions.

Using templates that allow repeated sequencing (e.g., circulartemplates) in a single reaction can increase the percent of a nucleicacid template for which nucleotide sequence data is generated, therebyproviding more complete data for further analysis, e.g., construction ofsequence scaffolds and/or consensus sequences for the nucleic acidtemplate. For example, each time a circular template is sequenced thetiming of the illuminated and non-illuminated periods can be reset tochange the regions of the template for which nucleotide sequence data isgenerated. As described above, the number of base positions separatingsequence reads generated in illuminated periods can be estimated basedon the temporal length of intervening non-illuminated periods and theknown rate of incorporation during the reaction and/or by the measuredrate of incorporation during the illuminated period(s). The known rateof incorporation can be based on various factors including, but notlimited to, sequence context effects due to the nucleotide sequence ofthe template nucleic acid, kinetics of the polymerase used, buffereffects (salt concentration, pH, etc.), and even data being collectedfrom an ongoing reaction. These factors can be used to determine theappropriate timing for the illuminated and non-illuminated periodsdepending on the experimental objectives of the practitioner, whether itbe maximizing length or depth of sequence coverage on a given templatenucleic acid, or optimizing sequence data collection from particularregions of interest. Alternatively, each time a circular template issequenced the timing of the illuminated and non-illuminated periods canbe kept the same to provide a greater-fold coverage of one or moreregions of interest in the template. Various methods for generatingredundant sequence reads are known in the art, and certain specificmethods are provided in U.S. Pat. Nos. 7,302,146; 7,476,503; U.S. Ser.No. 61/094,837, filed Sep. 5, 2008; U.S. Ser. No. 61/099,696, filed Sep.24, 2008; and U.S. Ser. No. 61/072,160, filed Mar. 28, 2008, all ofwhich are incorporated by reference herein in their entireties for allpurposes. A specific embodiment is also provided in the ExemplaryApplications section herein.

Another exemplary embodiment of an analysis of a plurality ofilluminated reactions using intermittent illumination comprises a firstilluminated period that is initiated at different times over theplurality of reactions. For example, the illuminated period for a firstreaction may start at 0 seconds, the illuminated period for a secondreaction may start at 5 seconds, the illuminated period for a thirdreaction may start at 10 seconds, and so forth. Additionally oralternatively, a first subset of reactions may begin at a first time, asecond subset may begin at a second time, etc. The first illuminatedperiod continues for a given length of time, followed by anon-illuminated period and a subsequent second illuminated period.Optionally, a plurality of non-illuminated periods and illuminatedperiods follow the first illuminated period. Staggered start times canprovide staggered data sets (e.g., two or more sequence reads) for theplurality of reactions, allowing multiple different stages of theoverall reaction to be interrogated in different reactions. Preferably,the staggered data sets overlap to an extent that allows furtheranalysis and validation of the reaction data. For example, asequencing-by-incorporation reaction subjected to such an embodiment ofthe invention would preferably have sufficient overlap between sequencereads from different individual reactions to allow construction of asequence scaffold and/or consensus sequence for a template nucleic acid.

A mask for use with a solid support (e.g., an array of confinements) canbe designed to allow illumination of one or more portions of the solidsupport while blocking illumination to other portions of the solidsupport. For example, a mask may comprise one or more windows that allowexcitation illumination to pass through the mask. Such a mask may bephysically moved over the surface of the solid support (or the solidsupport can be moved relative to the mask), e.g., to selectively allowexcitation illumination to reach a subset of confinements in an array.For example, a mask that allows 10% of reaction sites to be illuminatedcould be used to increase the sequencing scaffold coverage by slidingthe illumination area (the area being subjected to excitationillumination) back and forth across the solid support. The 10% ofreactions would cover certain regions of the nucleic acid template forany given time period (and therefore region of sequence in thetemplate). In certain embodiments, an automated mask that selectivelycontrols the timing of illumination of reactions on a solid supportduring the course of the reaction/acquisition may be used rather than amask that must be physically moved.

The timing of the illuminated and non-illuminated periods for a set ofreactions on a solid support may be the same or may vary, and may besynchronized or random. In certain embodiments in which the excitationillumination source is turned on and off, the timing of the illuminatedand non-illuminated periods for the set of reactions will be identical.In other embodiments, for example, those that comprise use of a mask,the timing of the illuminated and non-illuminated periods for the set ofreactions can vary so that while a subset of the reactions areilluminated, another subset of the reactions are not illuminated.Various exemplary and nonlimiting embodiments of masks that may be usedwith a set of reactions on a solid substrate are provided in FIGS. 3-5,as described below. In certain embodiments, theilluminated/non-illuminated status of each reaction may be random acrossthe solid support, e.g., to remove any experimental bias potentiallyintroduced by actively selecting which reactions to illuminate at agiven time, as long as the sequence reads being generated at theilluminated reactions and the time at which these reactions are notilluminated are able to be assigned to a particular reaction. For easeof discussion, the action of both illuminating and collecting emissionsignals from a reaction of interest, or a particular region on a solidsupport in which a reaction of interest is taking place, is referred toas “interrogating” that reaction and/or that region. A region being sointerrogated is termed an “observation region.”

FIG. 3 provides an exemplary embodiment of analysis of a plurality ofilluminated reactions using intermittent illumination and a mask. As inFIG. 2, an array of reactions on a solid support 310 is providedcontaining sixteen reaction sites, numbered for convenience (A). In B, amask 320 is provided with a single window 330 to allow passage ofillumination to a subset of reactions on the solid support. Window 330is wide enough to allow illumination of at least two columns of reactionsites on solid support 310. As in FIG. 2, a representation of theillumination data is shown in C, with bars extending across the graphindicative of illumination data collected for each reaction. Theposition of the bars relative to the x-axis provides the position of thesequence read relative to the template nucleic acid sequence, whichextends from position 0 (initiation of sequencing reaction) to n. Whenthe sequencing reaction is initiated at all positions on solid support310, the window 330 is positioned to allow illumination to onlyreactions 1, 5, 9, and 13, and these four reactions provide sequencereads 350 for the earliest stage of the reactions. The window 330 issubsequently moved to provide an illuminated period for reactions 2, 6,10, and 14 while still continuing the illuminated period for reactions1, 5, 9, and 13. The illumination data for reactions 2, 6, 10, and 14provides sequence reads 360, which partially overlap sequence reads 350for reactions 1, 5, 9, and 13. The window 330 is moved again to provideilluminated periods for reactions 3, 7, 11, and 15 while stillcontinuing the illuminated period for reactions 2, 6, 10, and 14, butremoving illumination from reactions 1, 5, 9, and 13. The illuminationdata for 3, 7, 11, and 15 results in sequence reads 370, which partiallyoverlap sequence reads 360 for reactions 2, 6, 10, and 14. A fourthposition of the mask 320 initiates an illuminated period for reactions4, 8, 12, and 16 while continuing illumination of reactions 3, 7, 11,and 15, but ending the illuminated period for reactions 2, 6, 10, and14. Sequence reads 380 correspond to sequence reads from reactions 4, 8,12, and 16. Finally, the window is moved to end the illuminated periodfor reactions 3, 7, 11, and 15 while continuing the illuminated periodfor reactions 4, 8, 12, and 16. Repeating the above process allows asecond read to be generated from each reaction, and this second read isnoncontiguous with the first read. For example, reactions 1, 5, 9, and13 correspond to reads 350 and, later in the reaction, reads 355. Thetwo reads generated in a single reaction do not overlap and areseparated by a length of nucleotides that was incorporated during thenon-illuminated period between the two illuminated periods.

The mask can optionally be passed over the substrate additional times togenerate additional reads until the reactions are complete or no longerprovide reliable data, such as when the total illumination time(computed by summing the times for the multiple illuminated periods) hassurpassed a photo-induced damage threshold period. Further, the mask maybe passed back and forth, or may pass over the solid support in only onedirection, e.g., always left to right, or vice versa.

Further, unlike the data shown in FIG. 2B which has gaps in the sequencecoverage for the template nucleic acid, the strategy provided in thisembodiment results in at least two-fold coverage across the entiretemplate nucleic acid (FIG. 3C), although at a lower-fold redundancy.The portion of the template covered by only reads 380 and reads 355 hasthe least-fold redundancy, and in some instances a gap in coverage maybe present in this region due to the movement of the mask 320 from thefar right to the far left of the solid support 310. Of course,oversampling by adding replicate reactions to the set of reactions, orusing templates that allow repeated sequencing (e.g., circulartemplates) in a single reaction can increase the coverage of a nucleicacid template, thereby providing more data for construction of sequencescaffolds and/or consensus sequences for the nucleic acid template.Various methods for generating redundant sequence reads are known in theart, and certain specific methods are provided in U.S. Pat. Nos.7,302,146; 7,476,503; U.S. Ser. No. 61/094,837, filed Sep. 5, 2008; U.S.Ser. No. 61/099,696, filed Sep. 24, 2008; and U.S. Ser. No. 61/072,160,filed Mar. 28, 2008, all of which have been previously incorporated byreference herein. The natural variation of polymerase translocation rateover the set of reactions is also apparent in this prophetic example asthe spreading of the sequence reads and decreasing overlap between readsfrom reactions in adjacent columns in the later stages of the reactionsas compared to the earlier stages.

FIG. 4A provides an embodiment of a mask similar to that provided inFIG. 3 except that it comprises three windows allowing multiplenonadjacent columns of reaction sites to be illuminated simultaneously.FIG. 4B provides an embodiment of a mask comprising twelve windows, eachof which allows illumination of a single reaction site on a solidsupport. The windows are oriented in the mask to allow illumination ofevery other reaction in each row and every other reaction in eachcolumn. It will be understood that these mask designs are merelyexemplary and nonlimiting embodiments as it is well within the abilitiesof the ordinary practitioner to determine an appropriate mask designdepending on the experimental design or the illuminated reactions to beinterrogated.

FIG. 5B illustrates yet another aspect of the instant invention in whichmultiple samples are analyzed on a single solid support usingintermittent illumination. Four different samples are disposed on asolid support, one in each quadrant 510, 520, 530, and 540 (A). A mask550 is used that comprises two windows 560 that allow multiple rows ofreaction sites to be illuminated simultaneously (B). A first position ofthis mask over a solid support in which two reactions in each quadrantare illuminated is demonstrated in C. A second position of the maskallowing illumination of the previously non-illuminated reactions isdemonstrated in D. The mask may be moved back and forth as indicated bythe double-arrow to provide multiple illuminated and non-illuminatedperiods for each reaction containing one of the four samples.

The present invention is also useful for redundant interrogation ofreactions or portions of a solid support of interest. In certainaspects, sequential interrogation of different observation regions maybe repeated a number of times, e.g., more than 2, 5, 10, 50, 100, 500,1000, or even more than 10,000 times. In general, this method ofstepping the observation region to another, preferably adjacent region,and repeating the interrogation process is generally referred to as a“step and repeat” process, and may be performed by various methods,including but not limited to moving the incident light and the solidsupport relative to one another and moving a mask across the surface ofthe solid support, as described above. Although described as a “step andrepeat” method, in some embodiments where the observation region ismoved across a substrate, that movement is not step-wise and iterative,but instead constitutes a continuous motion, substantially continuousmotion, or stepped movement, or an iterative motion whereby eachiterative step interrogates a new region that overlaps with some portionof the previously interrogated region. In particular, a substrate may bemoved continuously relative to an optical system, whereby theobservation region moves continuously across the substrate beinginterrogated (in a “scan mode”).

The present invention is optionally combined with an optical system thatprovides illumination and/or collection of emitted illumination.Preferably, the optical system is operatively coupled to the reactionsites, e.g., on a solid support. One example of a particularly preferredoptical system is described in U.S. Ser. No. 11/201,768, filed Aug. 11,2005, and incorporated herein by reference in its entirety for allpurposes. Optical systems are described further below.

In some embodiments, one or both of the solid support and optical systemare moved during interrogation. For example, a solid support beinginterrogated may be held stationary while the optical system is moved,or the solid support may be moved relative to a stationary opticalsystem. Such movement may be accomplished using any of a variety ofmanipulation hardware or robotic set-ups, e.g., a stepper/feederapparatus, and are well known in high performance printing technologiesand in the semiconductor industry. For example, robotic systems may beused to pick up and re-orient a given solid support in order tointerrogate different regions of the solid support, or make a previouslyunaccessible region (e.g., blocked by clips, support structure, or thelike) of the solid support accessible. Such robotic systems aregenerally available from, e.g., Beckman, Inc., Tecan, Inc., Caliper LifeSciences, and the like.

In addition to the foregoing, it will be appreciated that the reagentsin a given reaction of interest, including those reagents for whichphoto-induced damage is being mitigated in accordance with theinvention, may be provided in any of a variety of differentconfigurations. For example, they may be provided free in solution, orcomplexed with other materials, e.g., other reagents and/or solidsupports. Likewise, such reagents may be provided coupled to beads,particles, nanocrystals or other nanoparticles, or they may be tetheredto larger solid supports, such as matrices or planar surfaces. Thesereagents may be further coupled or complexed together with otherreagents, or as separate reagent populations or even as individualmolecules, e.g., that are detectably resolvable from other moleculeswithin the reaction space. As noted above, whether a particular reagentis confined by virtue of structural barriers to its free movement or ischemically tethered or immobilized to a surface of a substrate, it willbe described as being “confined.” Further examples of such confinedreagents include surface immobilized or localized reagents, e.g.,surface immobilized or associated enzymes, antibodies, etc. that areinterrogated upon the surface, e.g., through fluorescence scanningmicroscopy or scanning confocal microscopy, total internal reflectionmicroscopy or fluorometry, microscopy utilizing evanescent waves (see,e.g., U.S. Patent Publication Nos. 20080128627, filed Aug. 31, 2007;20080152281, filed Oct. 31, 2007; and 200801552280, filed Oct. 31, 2007,all of which are incorporated by reference in their entireties for allpurposes), surface imaging, or the like. For example, in some preferredembodiments, one or more reagents in an assay system are confined withinan optical confinement. Such an optical confinement may be an internalreflection confinement (IRC) or an external reflection confinement(ERC), a zero-mode waveguide, or an alternative optical structure, suchas one comprising porous film with reflective index media or aconfinement using index matching solids. More detailed descriptions ofvarious types of optical confinements are provided, e.g., inInternational Application Publication No. WO/2006/083751, incorporatedherein by reference in its entirety for all purposes.

The invention is generally applicable to any of a variety of opticalassays that require substantial illumination and/or photoactivatedconversion or excitation of chemical groups, e.g., fluorophores. Forexample, the compositions and methods provided herein may be used withfluorescence microscopy, optical traps and tweezers, spectrophotometry,fluorescence correlation spectroscopy, confocal microscopy, near-fieldoptical methods, fluorescence resonance energy transfer (FRET),structured illumination microscopy, total internal reflectionfluorescence microscopy (TIRF), etc. The methods provided herein may beparticularly useful in assays that are negatively impacted, directly orindirectly, by prolonged exposure to illumination. Of particularinterest are those assays that are impaired by the generation and/oraccumulation of triplet-state forms or free radicals duringillumination.

One particularly apt example of analyses that benefit from the inventionare single-molecule biological analyses, including, inter alia, singlemolecule nucleic acid sequencing analyses, single molecule enzymeanalyses, hybridization assays (e.g., antibody assays), nucleic acidhybridization assays, and the like, where the reagents of primary importare subjected to prolonged illumination with relatively concentratedlight sources (e.g., lasers and other concentrated light sources, suchas mercury, xenon, halogen, or other lamps) in an environment wherephotoconversion/excitation is occurring with its associated generationof products. In certain embodiments, the methods, compositions, andsystems are used in nucleic acid sequencing processes that rely ondetection of fluorescent or fluorogenic reagents. Examples of suchsequencing technologies include, for example, SMRT™ nucleic acidsequencing (described in, e.g., U.S. Pat. Nos. 6,399,335, 6,056,661,7,052,847, 7,033,764, 7,056,676, 7,361,466, 7,416,844, the fulldisclosures of which are incorporated herein by reference in theirentirety for all purposes), non-real-time, or “one base at a time”sequencing methods available from, e.g., Illumina, Inc. (San Diego,Calif.), Helicos BioSciences (Cambridge, Mass.), Clonal Single MoleculeArray™, and SOLiD™ sequencing. (See, e.g., Harris, et al. (2008) Science320 (5872):106-9, incorporated by reference herein in its entirety forall purposes.) Such prolonged illumination can negatively impact (e.g.,by introducing photo-induced damage) these reagents and diminish theireffectiveness in the desired reaction.

III. Prevention of Photo-Induced Damage

The methods provided herein are particularly useful in analyses thatutilize very limited concentrations of reactants, such as singlemolecule detection/monitoring assays. As will be appreciated, in suchreagent limited analyses, any loss, degradation, or depletion of acritical reagent will dramatically impact the analysis by furtherlimiting the reagent, which not only can adversely effect the detectablesignal, but may also directly impact the reaction being monitored, e.g.,by changing its rate, duration, or product(s). For example,photo-induced damage can include a photoinduced change in a givenreagent that reduces the reactivity of that reagent in the reaction,e.g., photobleaching of a fluorescent molecule, which diminishes orremoves its ability to act as a signaling molecule. Also included in theterm photo-induced damage are other changes that reduce a reactant'susefulness in a reaction, e.g., by making the reagent less specific inits activity in the reaction. Likewise, photo-induced damage includesundesired changes in a reagent that are caused by interaction of thatreagent with a product of another photoinduced reaction, e.g., thegeneration of singlet oxygen during a fluorescence excitation event,which singlet oxygen may damage organic or other reagents, e.g.,proteins. Photo-induced damage also includes downstream effects ofdamage to reactants, such as irreversible interactions between damagedreactants and other critical components of the reaction, e.g., reactiveproteins or enzymes. For example, damage to an enzyme that catalyzes areaction being monitored may cause a reduction in the rate of thereaction, in some cases stopping it altogether, or may reduce theduration or fidelity of the reaction.

As suggested by the foregoing, photo-induced damage generally refers toan alteration in a given reagent, reactant, or the like, that causessuch reagent to have altered functionality in a desired reaction, e.g.,reduced activity, reduced specificity, or a reduced ability to be actedupon, converted, or modified, by another molecule, that results from,either directly or indirectly, a photo-induced reaction, e.g., aphoto-induced reaction creates a reactant that interacts with and causesdamage to one or more other reactants. Typically, such photoreactiondirectly impacts either the reactant of interest, e.g., directphoto-induced damage, or impacts a reactant within one, two or threereactive steps of such reactant of interest. Further, such photoreactioncan directly impact the reaction of interest, e.g., causing a change inrate, duration, processivity, or fidelity of the reaction.

The amount of time an illuminated analysis may be carried out beforephoto-induced damage so substantially impacts the reactants to renderthe analysis non-useful is referred to as the “photo-induced damagethreshold period.” A photo-induced damage threshold period isassay-dependent, and is affected by various factors, including but notlimited to characteristics of enzymes in the assay (e.g., susceptibilityto photo-induced damage and the effect of such damage on enzymeactivity/processivity), characteristics of the radiation source (e.g.,wavelength, intensity), characteristics of the signal-generatingmolecule (e.g., type of emission, susceptibility to photo-induceddamage, propensity to enter triplet state, and the effect of such damageon the brightness/duration of the signal), similar characteristics ofother components of the assay. It can also depend on various componentsof the assay system, e.g., signal transmission and detection, datacollection and analysis procedures, etc. It is well within the abilitiesof the ordinary practitioner to determine an acceptable photo-induceddamage threshold period for a given assay, e.g., by monitoring thesignal decay for the assay in the presence of a photodamaging agent andidentifying a period for which the signal is a reliable measure for theassay. In terms of the invention, the photo-induced damage thresholdperiod is that period of illuminated analysis during which suchphoto-induced damage occurs so as to reduce the rate or processivity ofthe subject reaction by at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%,or 90% over the same reaction in the absence of such illumination. It isan object of the invention to increase the photo-induced damagethreshold period, thereby increasing the amount of time reactions canproceed toward completion with minimal damage to the reactants, therebylengthening the time in which the detectable signal is an accuratemeasure of reaction progression.

In some contexts, a “photo-induced damaged” reaction may be subject tospurious activity, and thus be more active than desired. In such cases,it will be appreciated that the photo-induced damage threshold period ofinterest would be characterized by that period of illuminated analysisduring which such spurious activity, e.g., as measured by an increase inreaction rate, or an increase in non-specific reaction rate, is no morethan 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% over anon-illuminated reaction. In one non-limiting example, where a nucleicacid polymerase, by virtue of a photodamaging event, begins toincorrectly incorporate nucleotides during template directed synthesis,such activity would impact the photo-induced damage threshold period asset forth above. In this case, the methods, devices, and systems of theinvention would increase the photo-induced damage threshold period, thusincreasing the amount of time the reaction could proceed before theabove-described spurious activity occurred.

With reference to nucleic acid analyses, it has been observed that intemplate-directed synthesis of nucleic acids using fluorescentnucleotide analogs as a substrate, prolonged illumination can result ina substantial degradation in the ability of the polymerase to synthesizethe nascent strand of DNA, as described previously, e.g., in U.S.Published Patent Application No. 20070161017, incorporated by referenceherein in its entirety for all purposes. Damage to polymerase enzymes,template sequences, and/or primer sequences can significantly hinder theability of the polymerase to process longer strands of nucleic acids.For example, reduction in the processivity of a polymerase leads to areduction in read lengths for sequencing processes that identifysequence constituents based upon their incorporation into the nascentstrand. As is appreciated in the art of genetic analysis, the length ofcontiguous reads of sequence directly impacts the ability to assemblegenomic information from segments of genomic DNA. Such a reduction inthe activity of an enzyme can have significant effects on many differentkinds of reactions in addition to sequencing reactions, such asligations, cleavages, digestions, phosphorylations, etc.

Without being bound to a particular theory or mechanism of operation, itis believed that at least one cause of photo-induced damage to enzymeactivity, particularly in the presence of fluorescent reagents, resultsfrom the direct interaction of the enzyme with photo-induced damagedfluorescent reagents. Further, it is believed that this photo-induceddamage of the fluorescent reagents (and possibly additional damage tothe enzyme) is at least partially mediated by reactive intermediates(e.g., reactive oxygen species) that are generated during the relaxationof triplet-state fluorophores. One or both of the photo-induced damagedfluorescent reagents and/or reactive intermediates may be included inthe overall detrimental effects of photo-induced damage.

In certain aspects, the invention is directed to methods, devices, andsystems that reduce the amount of photo-induced damage to one or morereactants during an illuminated reaction, e.g., thereby improving thereaction, e.g., by increasing the processivity, rate, fidelity,processivity, or duration of the reaction. In particular, methods areprovided that yield a reduction in the level of photo-induced damageand/or an increase in the photo-induced damage threshold period ascompared to such reactions in the absence of such methods, devices, andsystems. In particular embodiments, such methods comprise subjecting anilluminated reaction to periods of non-illuminated during the course ofthe reaction, as described above, or by temporarily removing componentsof the reaction mixture that are believed to cause such damage, asdescribed below.

As generally referred to herein, limited quantity reagents or reactantsmay be present in solution, but at very limited concentrations, e.g.,less than 200 nM, in some cases less than 10 nM and in still othercases, less than 10 pM. In preferred aspects, however, such limitedquantity reagents or reactants refer to reactants that are immobilizedor otherwise confined within a given area or reaction site (e.g., azero-mode waveguide), so as to provide limited quantity of reagents inthat given area, and in certain cases, provide small numbers ofmolecules of such reagents within that given area, e.g., from 1 to 1000individual molecules, preferably between 1 and 10 molecules. As will beappreciated, photo-induced damage of immobilized reactants in a givenarea will have a substantial impact on the reactivity of that area, asother, non-damaged reactants are not free to diffuse into and mask theeffects of such damage. Examples of immobilized reactants includesurface-immobilized or -localized reagents, e.g., surface-immobilized or-associated enzymes, antibodies, etc. that are interrogated upon thesurface, e.g., through fluorescence scanning microscopy or scanningconfocal microscopy, total internal reflectance microscopy orfluorometry, microscopy utilizing evanescent waves (see, e.g., U.S.Patent Publication Nos. 20080128627, filed Aug. 31, 2007; 20080152281,filed Oct. 31, 2007; and 200801552280, filed Oct. 31, 2007, all of whichare incorporated by reference in their entireties for all purposes),surface imaging, or the like. Various types of solid supports upon whichone or more reactants can be immobilized are described above.

In accordance with certain aspects of the invention, a reaction ofinterest within a first observation region is interrogated for one ormore illuminated periods that cumulatively are less than a photo-induceddamage threshold period, as set forth elsewhere herein. Suchinterrogation may occur coincident with or independent of interrogationof additional observation regions on a solid support containing thefirst observation region. In accordance with the present invention, theobservation region typically includes confined reagents (e.g., enzymes,substrates, etc.) that are susceptible to photo-induced damage, and mayinclude an area of a planar or other solid support upon which confinedreagents are immobilized. Alternatively or additionally, the observationregion may include a physical confinement that constrains the reagentsthat are susceptible to photo-induced damage, including, e.g.,microwells, nanowells, planar surfaces that include hydrophobic barriersto confine reagents.

In accordance with certain aspects of the invention, a reaction ofinterest within a first observation region is intermittentlyinterrogated under constant illumination by virtue of intermittentpresence of detectable components of the reaction, wherein the presenceof such detectable components has the potential to directly orindirectly cause photo-induced damage to one or more other reactioncomponents. For example, a buffer comprising detectable components of areaction can be temporarily replaced with a buffer comprisingnon-detectable versions of the same components of the reaction, therebyinterrupting data acquisition for the reaction. When data acquisition isto be recommenced, the buffer comprising detectable component issubstituted for the buffer comprising non-detectable components. Thissubstitution of reaction components may be repeated multiple times togenerate multiple sets of data collected at noncontiguous stages of thereaction. For example, such a substitution can occur at least about 2,4, 6, 8, or 10 times during the course of the reaction.

In certain preferred embodiments, the detectable components arefluorescently-labeled components that can be damaged by exposure toexcitation illumination, and can further cause damage to other reactioncomponents, as described above. For example, asequencing-by-incorporation reaction can be initiated in the presence offluorescently-labeled nucleotides whose incorporation is indicative ofthe nucleotide sequence of the nascent strand synthesized by apolymerase, and by complementarity, of the template nucleic acidmolecule. At a selected time point during the ongoing reaction, thelabeled nucleotides can be removed and replaced with unlabelednucleotides, for example, by buffer exchange. After a period of timeduring which data acquisition has been interrupted by the absence ofsignal from the ongoing reaction, the labeled nucleotides can bereintroduced to reinitiate data acquisition. The labeled nucleotides maybe removed and reintroduced multiple times and for various lengths oftime, as preferred by the ordinary practitioner. In this way, multiplenoncontiguous sequence reads can be generated from a single nucleic acidmolecule in real time.

The methods herein slow the accumulation of photo-induced damage to oneor more reagents, and may therefore indirectly mitigate the impact ofphoto-induced damage in an ongoing reaction of interest. By way ofexample, methods that reduce exposure of a critical enzyme component toillumination radiation (e.g., by subjecting the reaction to periods ofnon-illumination or by temporarily removing a component of the reactionresponsible for such damage) do not necessarily prevent thephoto-induced damage to the enzyme component, but rather extend thephoto-induced damage threshold period by slowing the accumulation ofphoto-induced damage in the reaction mixture. Measurements of reductionof photo-induced damage as a result of implementation of intermittentillumination may be characterized as providing a reduction in the levelof photo-induced damage as compared to a reaction subjected to constantillumination. Likewise, measurements of reduction of photo-induceddamage as a result of temporary removal of reaction componentsresponsible for such damage may be characterized as providing areduction in the level of photo-induced damage as compared to a reactionin which such components are present throughout. Further,characterization of a reduction in photo-induced damage generallyutilizes a comparison of reaction rates, durations, or fidelities,processivities, e.g., of enzyme activity, and/or a comparison of thephoto-induced damage threshold period, between a reaction mixturesubjected to such the methods and/or systems of the invention and areaction mixture not so subjected.

In the case of the present invention, implementation of the methods,devices, and systems of the invention generally results in a reductionof photo-induced damage of one or more reactants in a given reaction, asmeasured in terms of “prevented loss of reactivity” in the system. Usingmethods known in the art, the amount of prevented loss of activity canat least 10%, preferably greater than 20%, 30%, or 40%, and morepreferably at least 50% reduction in loss of reactivity or increase inprocessivity, and in many cases greater than a 90% and up to and greaterthan 99% reduction in loss of reactivity or increase in processivity. Byway of illustration, and purely for the purpose of example, whenreferring to reduction in photo-induced damage as a measure of enzymeactivity in the presence and absence of intermittent illumination, if areaction included a reaction mixture having 100 units of enzyme activitythat would, under constant illumination, yield a reaction mixture havingonly 50 units of activity, then a 10% reduction in photo-induced damagewould yield a final reaction mixture of 55 units (e.g., 10% of the 50units otherwise lost, would no longer be lost). Further, use of theinvention is expected to increase the performance (e.g., processivity,duration, fidelity, rate, etc.) of a reaction whose performance isnegatively impacted by constant exposure to illumination by at leastabout 2-, 5-, 10-, 20-, 30-, 50-, 80-, 100-, 500-, or 1000-fold overthat achieved by the reaction under constant illumination. For example,it is a specific object of the instant invention to increase theprocessivity of a polymerase enzyme in a sequencing reaction to allowcollection of data across a longer length of the template.

With regards to sequencing applications, the methods herein facilitatethe scaffolding of nucleic acid sequences in reactions susceptible tophoto-induced damage. For example, if the sequencing device has 1000base pair average readlength under constant illumination, one couldsubject the reaction to illuminated periods timed to allow approximately100 nucleotides to be incorporated into the nascent strand of read,followed by non-illuminated periods timed to allow approximately 1000nucleotides to be incorporated “in the dark.” The sequence readsresulting from this experimental design would comprise about tensequence reads of about 100 nucleotides each separated by gaps of about1000 nucleotides each. If a plurality of sequencing reactions werecarried out in this manner, and the illuminated periods were staggeredappropriately, the reads from the plurality of reactions could becombined to provide nucleotide sequence data for the entire templatenucleic acid. This would potentially allow sequence scaffolds to bebuilt much more easily than can be done with short-read systems,enabling structural analysis of previously impossible-to-sequencesections of highly repetitive DNA, given the sequencing system iscapable of long reads in the absence of photodamage.

IV. Software and Algorithm Implementations

The methods herein may operate with numerous methods for sequencealignment including those generated by various types of known multiplesequence alignment (MSA) algorithms. For example, the sequence alignmentmay comprise one or more MSA algorithm-derived alignments that aligneach read using a reference sequence. In some embodiments in which areference sequence is known for the region containing the targetsequence, the reference sequence can be used to produce an MSA using avariant of the center-star algorithm. Alternatively, the sequencealignment may comprise one or more MSA algorithm-derived alignments thatalign each read relative to every other read without using a referencesequence (“de novo assembly routines”), e.g., PHRAP, CAP, ClustalW,T-Coffee, AMOS make-consensus, or other dynamic programming MSAs.Depending on the sequence-generating methods used, the determination ofsequence alignment may also involve analysis of read quality (e.g.,using TraceTuner™, Phred, etc.), signal intensity, peak data (e.g.,height, width, proximity to neighboring peak(s), etc.), informationindicative of the orientation of the read (e.g., 5′→3′ designations),clear range identifiers indicative of the usable range of calls in thesequence, and the like. Additional algorithms and systems for sequencealignment are well know to those of skill in the art, and are describedfurther, e.g., in G. A. Churchill, M. S. Waterman (1992) “The Accuracyof DNA Sequences: Estimating Sequence Quality,” Genomics 14: 89-98; M.Stephens, et al. (2006) “Automating sequence-based detection andgenotyping of SNPs from diploid samples,” Nat. Genet., 38: 375-381; J.Hein (1989) Mol. Biol. Evol., 6: 649-668; U.S. Ser. No. 12/134,186,filed Jun. 5, 2008; and U.S. Ser. No. 61/116,439, filed Nov. 20, 2008.

A standard sequence alignment problem in the context of DNA sequencingis to align the sequence of a relatively short fragment (<2 kilobases)to a large target sequence. The assumption is made that this fragmentrepresents a contiguous portion of DNA to be mapped to a single locationon the reference sequence. (A “contiguous portion” to be mapped to asingle location may contain small insertions and/or deletions and stillbe considered contiguous in this context.) With the further developmentof nucleic acid sequencing technologies (e.g., from Illumina, Inc. (SanDiego, Calif.), Helicos BioSciences (Cambridge, Mass.), and AppliedBiosystems, Inc. (Foster City, Calif.)) and mate-pair sequencingprotocols (see, e.g., U.S. Patent Pub. No. 2006/0292611 A1, which isincorporated by reference herein in its entirety for all purposes), thealignment problem has been extended to align two fragments coming fromthe same read to the reference sequence using some knowledge of theexpected mate-pair configuration (distance and orientation).

With regards to mate-paired reads, mapping two fragments with a distanceconstraint and orientation constraint has been treated by variousshort-read mapping algorithms, e.g., SOAP (Li, et al. (2008)Bioinformatics, 24, 713-714); SOAPdenovo; and Maq, a set of programsthat map and/or assemble fixed-length Solexa/SOLiD reads (SourceForge,Inc.). While these algorithms can handle simple cases of mate-pairalignment, which generally treat the specific problem of only two readscoming from a mate-paired sequence and use the distance constraint as ahard filter (i.e., if two reads are within x bp of each other and in thecorrect orientations, report them as a mate-pair hit), the methodsprovided herein are more general and can handle much more complex datasets, including those with multiple reads, those for which a referencesequence is or is not present, potential non-template sequence (e.g.,adapter regions or linker portions described below), and complexdistance and orientation constraints. Other programs are also availablethat attempt to generalize on top of the mapping and aligning performedby the programs described above. These include, e.g., Breakdancer,variationhunter, GASV, etc., which can handle more complex mappings,e.g., by clustering.

Real-time single molecule sequencing presents opportunities forobtaining much more complex sequence fragments from a single DNAsequencing read. Two examples are the reading of multiple discontiguoussequence fragments from a single long stretch of DNA using a pulsed orintermittent detection system (e.g., intermittent illumination) asdescribed herein and the contiguous reading of forward, reverse andadapter fragments from a circular templates (SMRTbell™ templates; seee.g., U.S. Ser. No. 61/099,696, filed Sep. 24, 2008; U.S. patentapplication Ser. No. 12/383,855, filed Mar. 27, 2009 and U.S. patentapplication Ser. No. 12/413,258, filed Mar. 27, 2009, all of which areincorporated by reference herein in their entireties for all purposes).Further, methods for sequencing template nucleic acids comprisingmodifications, including detecting kinetic signatures of suchmodifications during single-molecule sequencing reactions, are providedin U.S. Patent Application Nos. 61/201,551, filed Dec. 11, 2008;61/180,350, filed May 21, 2009; and Ser. No. 12/945,767, filed Nov. 12,2010; and U.S. Patent Publication No. 2010/0221716, the disclosures ofwhich are incorporated herein by reference in their entireties for allpurposes.

Certain aspects of the invention provide methods for optimally aligningsuch sequences to a reference sequence using knowledge of the molecularconfiguration and/or sequencing protocol used to generate the relatedsequence reads. In particular, methods are provided to address thegeneral problem of mapping multiple fragments to a reference sequencewith variable distance and orientation constraints.

Beginning with raw sequence data generated by a nucleic acid sequencinginstrument (step 1), the sequence data is mapped to a target sequence(step 2) using a local alignment method which produces sub-optimal localalignments as well as the optimal alignment, for example, theSmith-Waterman algorithm. Another, more flexible example of a localalignment method is a chaining method using a method for aligning veryshort fragments to the target sequence (e.g., kmer-indexing, suffixtrees, suffix arrays, etc.) and chaining the resulting hits back intolonger chains of significant matches (see, e.g. D. Gusfield, Algorithmson Strings, Trees, and Sequences, Cambridge University Press: Cambridge,UK, 1997, which is incorporated by reference herein in its entirety forall purposes). The chains do not necessarily need to be refined bydynamic programming in order to be useful for the following algorithm,permitting a very fast algorithm. In certain embodiments,dynamic-programming refinement of the chain might improve the power(area under the ROC curve) of the algorithm.

The target sequence consists of the potential hypotheses for themolecular template in question. In the example of nucleic acidsequencing methods using iterative illumination for sequencing a shotgunfragment from a linear DNA sequence, the potential hypotheses are bothorientations of the genome (since we do not know the originalorientation of the fragment). In the example of sequencing of aSMRTbell™ template (e.g., see Example 1 herein), the hypotheses includeboth orientations of the genome and known adapter sequences. Theparameters determining how many hits are reported for each localfragment can be varied to change the specificity and sensitivity of thisalgorithm. FIG. 6 shows what these hits might look like for a SMRTbell™template (represented as paths in the sequence alignment matrix, whichis often called the dynamic-programming matrix, although it isn'tnecessary to use dynamic programming to find these paths).

After the potential local alignments have been enumerated, a weighteddirected graph is constructed with each local alignment represented as anode in the graph (step 3). The edges are drawn between nodes if theyrepresent a potential reconstruction of the original molecular templateusing knowledge of the expected molecular configuration. The directedconnection of an alignment path A to an alignment path B is interpretedas “The target sequence represented by B could follow the targetsequence represented by A in the original molecule.” For example, if alinear single-stranded DNA molecule is being sequenced by a method thatuses iterative illumination, then fragments from opposite orientationswould not be expected to be connected (unless the linear single-strandedDNA molecule also included oppositely oriented sequences, e.g., as inthe case of a linearized SMRTbell™ template.) In general, fragments thatrepresent the same stretch of the sequencing read but that align todifferent regions on the target sequence would not be connected. Asidefrom these examples, the rules for connecting nodes should be fairlyloose to permit exploration of weak possibilities that gain significancewhen all the evidence (e.g. all the sequence reads) are considered. Theassignment of edge weights handles the proper weighting of thelikelihood of these edges, and the speed of the algorithm can be tunedby optimizing the pruning of highly unlikely edges. As usual thisrepresents a tradeoff between speed and sensitivity.

Weights are assigned to connections (A→B) in the graph representing thelog-likelihood that target fragment A is followed by target fragment Bin the original molecule.

w(A→B)=−log P(B|A)

The conditional probability P(B|A) encodes the knowledge of the possiblemolecular configurations and the alignment significance of B.

P(B|A)=f(B)g(A,B)

where f is a measure of alignment significance (either theoretical orempirically obtained) and g encodes the physical constraintsrepresenting the allowed molecular configurations.

For example, in the context of sequencing using iterative illuminationthe following may be known: the time between the end of one fragment andthe beginning of the next fragment is 200 seconds. If the polymeraseincorporates bases with an average rate of 4 bp/sec with a standarddeviation of 1 bp/sec, it can be hypothesized that the probability oftarget fragment 2 following target fragment 1 is determined by thedistance between these fragments on the target and a normal probability:

${g\left( {A,B} \right)} = {\frac{1}{\sqrt{2{\pi\left( {200} \right)}}}{\exp\left\lbrack {{{- \left( {d - {800}} \right)}/2}\left( {200} \right)^{2}} \right\rbrack}}$

In a SMRTbell™ template example, knowledge of the expected insert sizeand the observed distance and orientation between fragments would beused to weight the likelihood that these two fragments could come from acorrectly generated SMRTbell™ template. This weight could include theexpected rate of the polymerase as well and rules for the orientation offragments with respect to each other and their distance apart in theoriginal read. For example, while it may be expected that two forwardfragments mapping to the same region in the target genome potentiallycome from multiple passes around a SMRTbell™ template molecule, thosefragments would not be expected to be immediately adjacent in sequencingtime. The weighting function would account for the proper amount ofexpected time between such fragments (i.e. the elapsed time would beexpected to be long enough to include two adapter sequences and areverse sequence).

In general, the weighting function could be arbitrarily complex andtuned to empirically observed relationships between sequencing fragmentsgiven the available knowledge (distance between fragments on the targetsequence, sequencing time between fragments, expected length of thetemplate, etc.). For example, the empirical probability distributionsmight be observed to exhibit longer tails than a Gaussian probabilitymodel might predict. The use of a conditional log-likelihood for theassignment of edge weights is motivated by the following logic. In agraph of possible local alignments it is desirable to find a highlylikely path that best explains the observed data. Consider a paththrough three nodes A, B, and C, with P(ABC) being the probability thatABC is the correct assignment:

$\begin{matrix}{{P\left( {ABC} \right)} = {{P\left( C \middle| {AB} \right)}{P\left( B \middle| A \right)}{P(A)}}} \\{\approx {{P\left( C \middle| B \right)}{P\left( B \middle| A \right)}{P(A)}}}\end{matrix}$

where the last approximation is justified by the observation that theconstraints between allowable assignments to the target sequence aretypically local in nature. Generalizing this formula for a path a₁, . .. , a_(N) and taking the negative logarithm of both sides gives

${- {\log{P\left( {a_{1}\mspace{14mu}\ldots\mspace{14mu} a_{N}} \right)}}} = {{- {\sum\limits_{i = 1}^{N - 1}{\log{P\left( a_{i + 1} \middle| a_{i} \right)}}}} - {\log{P\left( a_{1} \right)}}}$

It is apparent that the edge weights are additive if we uselog-likelihood and we can use standard shortest-path algorithms fordirected graphs to find the optimal path. A hypothetical directed graphis illustrated in FIG. 7. This graph corresponds to the situationdepicted by the alignments pictured above. Heavier lines correspond tomore likely paths with the optimal path shown in blue. Dashed linesrepresent forbidden transitions. Not all paths are considered in theillustration to avoid clutter in the presentation. The general formulalisted above includes a “one-body” term P(a₁) for the starting node ineach path that weights the probability that this initial alignment iscorrect. To accommodate this probability in a path-finding algorithm weadd a pseudo-source s to the graph which connects to every possible node(not shown in the graph above). The edge weight connecting thepseudo-source with a node a_(i) is −log P(a_(i)) This allows the use ofa conventional single-source shortest-path algorithm starting from thepseudo-source. The desired probability P(a_(i)) can come from a measureof alignment significance (theoretical or empirically determined) orcould be set uniformly across all alignments to allow the path logic todetermine the best path assignment, independent of the relative value ofthe starting points. It is anticipated that a threshold will be requiredhere to only allow edges between the pseudo-source and nodes for highlylikely alignments; otherwise the shortest path algorithm in the nextstep will not give the desired path.

After construction of the weighted directed graph, the shortest path toeach node is determined (step 4). The graph is directed and acyclic(DAG) so we can use the standard shortest-path DAG algorithm (see T. H.Cormen, C E Leiserson, R L Rivest, Introduction to Algorithms, MITPress: Cambridge, Mass., 1990). This algorithm scales as O(V+E) andshould be very quick for these graphs. After the shortest path to eachnode is determined, the paths need to be ranked to declare the bestassignment. It is suggested that the best metric would be a measurewhich rewards paths that explain more of the sequenced read (longerpaths) with high likelihood. One such metric would be the normalizednegative log-likelihood: dividing the total weight of the path by thenumber of bases in the sequenced read explained by this path. For morecomplicated graphs or edge-weight assignments, Dijkstra's algorithm, theBellman-Ford algorithm, or the A* algorithm could be applied. Otheralgorithms that may also be used include, but are not limited to theFloyd-Warshall algorithm.

For noisy sequence data it is likely that the local alignments found instep 2 will occasionally overlap with each other in the sequenced readeven though it is physically impossible for such overlaps to occur in aperfect system (unless there has been a rearrangement relative to thereference genome). As such, some amount of slack must be allowed in theedge assignment logic in step 3 to account for not knowing the preciseboundaries of each local alignment. Once the best physical modelexplaining the observed read is determined, the boundaries of the localalignments can be refined to reflect the physical necessity that eachbase in the sequenced read can only be represented in one localalignment. It is also desirable to explain all of the bases in betweenthe local alignments that haven't been assigned in the graph. Onestraightforward approach to refinement would be to construct the perfectmodel of the sequence and to realign the sequenced read to thissequence. This refinement algorithm would preserve physical constraints(each base in the sequenced read can only be explained by one locationin the template) and would assign all bases between the extremal nodesin the optimal path.

Certain aspects of the software and algorithm implementations describedherein may be varied or altered without departing from the spirit andscope of the invention. For example, with regards to algorithm seeding,many algorithms can be applied for the original determination ofsub-optimal local alignments (step 2). Conventional examples includeFASTA, BLAST, or Smith-Waterman. It is expected that the best benefitwill be obtained from using short-sequence alignment algorithms (suffixarray, suffix tree, Boyer-Moore, Rabin-Karp, kmer-indexing, and thelike) followed by chaining to establish regions of significant matches.An advantage of the algorithm described here is that it does not requiredynamic-programming refinement of the resulting chains and therefore canbe quite fast, however it is expected that using dynamic programming torefine the chains in step 2 could increase the power of the algorithm.

With regards to graph construction, there will be advantages to tuningthe logic of edge assignments to keep the size of the graph manageable.It is possible that steps 2 and 3 might be combined to in a greedyfashion to focus the potentially slow step 2 into productive areas ofthe graph. For example, if a particularly strong hit is found early instep 2, then it may be beneficial to search for sub-optimal hits only inthis local vicinity, knowing that this strong hit should be in the finalsolution. Tuning of the graph construction might include thresholds,below which edges are not created. Further, there are multipleparameters (minimum chain length, minimum probability for edgeassignment, relative weighting of length vs. accuracy, etc.) which canbe exposed and tuned in this algorithm to maximize the sensitivity andspecificity of the algorithm for a given scenario.

With regards to determination of the distance a polymerase travelsbetween reads, various strategies are provided that are moresophisticated than estimation based upon the rate of incorporation andthe time between detection (e.g., illuminated) periods. In certainembodiments, the distribution of the base pair distance travelled by apolymerase during a non-detection period is called p(x). Thedistribution of enzyme velocities, p(v), is estimated by aligningobserved reads to a reference sequence, and this distribution isrepresented as the number of reference bases per unit time. There is alength of time, τ, over which measurement of the instantaneous rate isnot independent. While this method of determining the distance thepolymerase travels during a non-detection period should not be overlysensitive to non-independent estimation of the polymerase rate, it islikely to strive for independent measurements of the rate. The distanceτ can be estimated from an exponential fit to the auto-correlationfunction <δv(t)δv(t+Δt)>, and v(t) tabulated across the aligned sequenceat increments of τ.

Where multiple single polymerase enzymes are being observedsimultaneously, e.g., each being optically resolvable from every otheron a single array, the p(v) for each is preferably determinedindependently for each enzyme. Further, information regarding rare butextended events, such as polymerase “stalling” on the template, can bemeasured across a larger data set. For example, the statistics of stallscan be determined by aggregating rate measurements across an entirearray. Where a stall distribution is characterized by a “long tail”corresponding to multi-exponential behavior of IPD distribution, such adistribution of polymerization rates can be extended for stalls longerthan the observed reaction by fitting the long-tail behavior to anappropriate functional form, e.g., using a single-exponential parametricmodel or other physically motivated model (e.g., multi-exponential,stretched exponential, power-law, etc.) In certain preferredembodiments, the following representation of a “per-enzyme” p(v) isused:

${p(v)} = \frac{{{f(v)}{p_{enzyme}(v)}} + {\left\lbrack {1 - {f(v)}} \right\rbrack_{p_{a{rray}}}(v)}}{{\int{{f\left( v^{\prime} \right)}{p_{enzyme}\left( v^{\prime} \right)}}} + {\left\lbrack {1 - {f\left( v^{\prime} \right)}} \right\rbrack_{p_{array}}\left( v^{\prime} \right){dv}^{\prime}}}$

where f(v) is an interpolating function designed to retain informationabout the zero-velocity tail of the global p(v) distribution whiletaking the estimate of the polymerase velocity dynamics (e.g., thedominant high velocity mode) from the specific enzyme. Such aninterpolating function is:

${f(v)} = \frac{1 + {{erf}\left( {v/v_{0}} \right)}}{2}$

where v₀ is a scale parameter to be chosen based on experience (butoptionally fixed). Alternatively, the average of the empiricalp_(enzyme)(V) and p_(array)(V) can be used. This approach can bemotivated by a Bayesian approach to density estimation. Other kerneldensity and Bayesian methods can be suggested. Alternatively or inaddition, the robustness of p(v) to conditions and daily phenomenon canbe explored and used to estimate p(v) more globally, e.g., using one ormore weekly control experiments.

Given the lack of a known reference sequence for de novo assembly,several alternative ways to formulate p(v) are provided as follows. Forexample, in a first embodiment a control template (essentially a proxyreference sequence) can be subjected to sequencing, e.g., in the samereaction as the de novo sample or in an identical reaction. The observedvelocity for the sequencing reactions would be measured based uponalignments of the reads from the control template to its known sequence.Typically, a per-enzyme correction would not be available for the p(v)and p(v) would default to an array-averaged p(v). In a secondembodiment, a previously determined p(v) from experiments using a knownreference sequence can be used, e.g., where the previous experimentswere performed under the same conditions as the de novo experiments. Ina third embodiment, p(v) is estimated by using qualityinformation/metrics to screen for the most likely “true” calls, andrestricting the estimates of v to regions containing those calls. In afourth embodiment, where error is low, the called base rate andreference base rate converge to the same rate, and measurements of p(v)without knowledge of the reference become substantially reliable.Further, even if they do not fully converge, they can still be used toaccurately infer p(v), as long as the called base rate is predictablyhigher/lower than the reference base rate. Yet further, the measurementof p(v) when a reference sequence is not available can benefit from adetailed look at the probability model which is available from analgorithm like a CRF. That is, p(v) can be tabulated using a weightedsum over paths through a CRF probability model.

As will be clear to the ordinary practioner based upon the teachingsherein, this framework extends naturally to the measurements of otherpotentially systematic variations in p(v) across an array, e.g., evenwhere a single reaction mixture is applied to the entire array. Forexample, the local temperature of the reaction environment can varysystematically across an array of reactions. The average and variationin the rates of polymerase enzymes on the array would likely have adependence on this hidden variable. Where the functional form of thetemperature dependence is known, the measurement of p(v) can bestabilized across the array by modeling a de novo p(v) asp_(cond)(v)+p_(x,y)(v) where x,y are geometrical variables defining thelocation on the array. Further, p(v) has been found to be somewhatvariable over time. As such, in certain embodiments a model of p(v;t) isdeveloped using an appropriate model for the evolution of p(v) overtime.

Once a representative distribution of velocities p(v) has been obtainedfor a given read from a given reaction, the expected travel distance inthe non-detection period can be expressed as:

${p\left( \frac{x}{\tau} \right)} = {I{L\left\lbrack {{L\left\lbrack {p(v)} \right\rbrack}^{t/\tau}\frac{1 - {L\left\lbrack {p(v)} \right\rbrack}}{s}} \right\rbrack}}$

where L[ ] and IL[ ] stand for the Laplace and inverse Laplacetransform, respectively. A similar result is derived in Svoboda, et al.(PNAS 91:11782 (1994)) and readily follows from considering the pdf of asum of random variables. Optionally, in certain embodiments densityestimation techniques (e.g., kernel density estimation, etc.) are usefulwhen modeling p(v) since they can smooth the resulting numericalcalculations in the Laplace and inverse Laplace transform.

Knowledge of the complete distribution has several advantages over thecommonly applied Gaussian approximation. For example, knowledge of thecomplete distribution of insert lengths is very desirable when using aBayesian framework approach to detect structural variation. (See, e.g.,Bashir, et al. (2008) PLoS Comput. Biol. 4:51; Hormozdiari, et al.(2009) Genome Res. 19:1270; and Lee, et al. (2008) Bioinformatics24:59.) While Bashir, et al. does not strictly follow a Bayesianapproach, the geometric approach described in the paper can bestraightforwardly modified to incorporate an actual posterior instead ofthe boxcar posterior assumed in the paper. Further, during mapping ofnoncontiguous reads to a genome where they are expected to be concordant(i.e., not a structural variation), it is useful to consider the knowndistribution when judging the significance of the resulting alignmentsbetween the observed reads and the genomic sequence. In addition, whenclustering noncontiguous reads that scaffold contigs in a de novoassembly, a path of Bayesian significance can be followed that is verysimilar to that followed in the structural variation case discussedsupra.

In further embodiments, the determination of the distance a polymerasetravels between reads is performed using an algorithm based on asimulation approach rather than the exact analytical result used in thealgorithm described above. This method relies on Monte Carlo samplingfrom a distribution, which allows a better extension to arbitraryempirical distributions. It also lacks the difficult computations ofnumerical Laplace and inverse Laplace transforms, and permitscalculation of distances traveled during non-detection periods when theunderlying kinetic processes have multi-phasic kinetics, e.g., thepresence of long stalls.

This approach aims to calculate the distribution of the distance xtravelled by an enzyme during a time t during which it was not beingobserved (e.g., during a non-detection period). In some embodiments, adistribution of local rates, p(v), is estimated, where the definition of“local” is set by the correlation length of the rate autocorrelationfunction, e.g.:

$\left\langle {\delta\;{v(t)}\delta\;{v\left( {t + \Delta} \right)}} \right\rangle\text{∼}{\exp\left( \frac{- \Delta}{\tau_{corr}} \right)}$

Given a local rate distribution and an assumption that independentidentically distributed (i.i.d.) draws can be made from thisdistribution, one approach to calculating the distribution is asfollows. First, draw N=t/τ_(corr) velocities from p(v); and subsequentlysum them and record them as an estimate of x/τ_(corr). Repeat theprocess M times, with the optimal choice of M dependent on the desiredlevel of precision for estimation of the p(x) distribution. In certainpreferred embodiments, M is between about 1000 and about 5000, e.g. atleast about 1000, 2000, 3000, or 4000, or is about 5000.

In some embodiments in which the enzyme system is not well explained bya single kinetic process or cycle (as in the case of observed stallingbehavior), above-described rate autocorrelation function and the i.i.d.assumption will be violated. As such, a probability model having aricher structure is preferably used. One such probability model is aHidden Markov Model (HMM). FIG. 15 provides an exemplary illustration ofan HMM for modeling a simple “pausing” vs. “sequencing” system. Wherethe kinetics of the pausing state can be well described by asingle-exponential, this model is expected to describe the observeddistribution of local velocities. The single-exponential assumption isimplicit in the state structure of the model since the amount of timespent in the pause state will be a geometric distribution with meanp/(1−p) [i.e., the observed stall times will have to be added to thismodel]. If the stall kinetics are multi-phasic, then more “dark states”will have to be added to this model. Further, the model shown in FIG. 15can actually be treated as a Markov Model and not a Hidden Markov Modelwithout much loss of generality because the “pause” state is notactually hidden due to the fact that the data collected during the pausestate is highly distinguishable from the data collected during thesequencing state. As such, the general HMM apparatus is not necessary.The model in FIG. 15 can be used to simulate the distribution of localvelocities when there is a long-term pause or stall phase present in thereaction data kinetics. S0 is the start state, and there is no explicitend state since this model is used as a generative model and it isassumed that it is run forward for a prescribed number of steps. Thequalities P(P→S) and P(S→P) represent exit from a stalled state andentry into a stalled state, respectively. These qualities can bemeasured by an EM algorithm or they can be quickly estimated by physicalobservables.

$P_{P\rightarrow S} = {1/\left( {1 + \frac{\tau_{stall}}{\tau_{corr}}} \right)}$

and P(S→P) is the frequency of stall starts per τ_(corr). (Exampleparameters are τ_(stall)=80 seconds; τ_(corr)=10 seconds; andP(S→P)=1/24.) The simulation estimate of p(x) can now be produced usingthe procedure outlined above in which N=τ_(corr) velocities are drawnfrom p(v); and they are subsequently summed and recorded as an estimateof x/τ_(corr). The process is repeated M times, with the optimal choiceof M dependent on the desired level of precision for estimation of thep(x) distribution. In certain preferred embodiments, M is between about1000 and about 5000, e.g. at least about 1000, 2000, 3000, or 4000, oris about 5000. FIG. 16 shows exemplary simulated applications of thismethod. FIG. 16A shows a sample of velocities drawn from the HMM in FIG.15 with the parameters P(S→P)=1/24; P(P→S)=1/11; andp(v)˜Gamma(48,0.25). FIG. 16B illustrates a resulting histogram of localvelocities. FIG. 16C provides an estimated distance traveled during a1300 second non-detection period, which is calculated by sampling 2000estimates from the HMM model.

FIG. 17 provides an illustrative example of two observed histograms ofdistances traveled during a non-detection period. The influence ofpause/stall behavior can be seen in the heavy-left tailing of bothdistributions.

While the simulation method in which i.i.d. draw assumption is valid ismore general and can treat arbitrary p(v) and more complex models fornon-sequencing states, the two-state model using the HMM can be treatedanalytically. The result of this is:

${p\left( {x/\tau_{corr}} \right)} = {\sum\limits_{N_{S} = 0}^{N}{{\pi_{N_{S}}(x)}{p_{N}\left( N_{S} \right)}}}$

where π_(N) _(s) (x) is the distribution of the sum of Ns variablesdrawn from p(v). For the general case, this distribution is given by theLaplace transform approach presented above. For p(v)˜Normal(μ,σ), thisdistribution is distributed as Normal(N_(S)μ, √N_(S)σ). Forp(v)˜Gamma(k,θ), this distribution is distributed as Gamma(N_(S)k,θ).P_(N)(N_(S)) is the number of cycles spent in the sequencing state if weobserve N cycles from the Markov process in FIG. 15. The expression forthis is described in Pedler, et al. (1971) J. Appl. Prob. 8:381, whichis incorporated herein by reference in its entirety for all purposes.

As will be clear to one of ordinary skill in the art upon review of theteachings herein, these methods can be readily extended to thenon-detection period estimations of procession by other cyclicalbiological reactions, such as the action of reverse transcriptase or thesynthesis of proteins by a ribosome complex, e.g., and certain preferredembodiments of such reactions are further described in U.S. Ser. No.12/767,673, filed Apr. 26, 2010; and U.S. Ser. No. 12/813,968, filedJun. 11, 2010, the disclosures of which are incorporated herein byreference in their entireties for all purposes. Further, the simulationmodel described above is not restricted to simple two-state kinetics,and the use of p(v) is not restricted to analytical models. In fact, incertain embodiments, empirical estimates are preferably used.

Although useful in certain preferred embodiments of the invention,certain algorithms as presented above do not easily handle the casewhere the template does not match a physically-motivated expected model.A relevant example of such a case is when the template contains agenomic structural variation (SV), such as translocation, whereby twofragments which are correctly adjacent in the template are located veryfar apart in the reference genome. Such structural variation cases arebest handled in the context of the current algorithm by reporting theconfidence of an observed path and reporting situations when nophysically expected path seems to fit the observed data. In general, thedetection of structural variation requires the presence of multiplehighly significant local alignments which can be identified assignificantly overturning the null hypothesis of matching the genomicordering of fragments with their own individual merit. Nevertheless,with molecular redundant sequencing such as SMRTbell™ templatesequencing the current algorithm can be adapted to improve the abilityto identify an SV event. Such a modification could be a feedbackapproach which allows modification of the linking constraints in step 3to allow very far separations on the target sequence when the individualalignments are very significant. Only one such highly-significant pairwould be needed to enable the rescue of less significant partial matchesthat support the same SV hypothesis.

The software and algorithm implementations provided herein areparticularly suited for transforming sequence read data generated fromvarious sequencing technologies (e.g., sequencing-by-synthesis,intramolecular redundant sequencing, Sanger sequencing, capillaryelectrophoretic sequencing, pyrosequencing, ligase-mediated sequencing,etc.) into consensus sequence data that provides a representation of theactual nucleotide sequence of the template nucleic acid that wassubjected to the sequencing reaction(s) from which the sequence readdata was generated. The software and algorithm implementations providedherein are preferably machine-implemented methods. The various stepsrecited herein are preferably performed via a user interface implementedin a machine that comprises instructions stored in machine-readablemedium and a processor that executes the instructions. The results ofthese methods are preferably stored on a machine-readable medium, aswell. Further, the invention provides a computer program productcomprising a computer usable medium having a computer readable programcode embodied therein, the computer readable program code adapted toimplement one or more of the methods described herein, and optionallyalso providing storage for the results of the methods of the invention.

In another aspect, the invention provides data processing systems fortransforming sequence read data from one or more sequencing reactionsinto consensus sequence data representative of an actual sequence of oneor more template nucleic acids analyzed in the one or more sequencingreactions. Such data processing systems typically comprise a computerprocessor for processing the sequence read data according to the stepsand methods described herein, and computer usable medium for storage ofthe initial sequence read data and/or the results of one or more stepsof the transformation (e.g., the consensus sequence data).

While described with reference to certain specific applications above,it will be understood that these methods are also applicable to othertypes of complex data sets, and the invention should not be limited toonly the specific examples provided herein. Other applications of theinstant methods will be clear to those of ordinary skill in the art andare considered to be additional aspects of the instant invention.

V. Devices and Systems

The invention also provides systems that are used in conjunction withthe compositions and methods of the invention in order to provide forintermittent detection of analytical reactions. In particular, suchsystems typically include the reagent systems described herein, inconjunction with an analytical system, e.g., for detecting data fromthose reagent systems. For example, a sequencing reaction may besubjected to intermittent illumination, and the sequencing system mayinclude the system components provided with or sold for use withcommercially available nucleic acid sequencing systems, such as theGenome Analyzer System available from Illumina, Inc., the GS FLX System,available from 454 Life Sciences, or the ABI 3730 System available fromLife Technologies, Inc.

In certain preferred embodiments, reactions subjected to intermittentillumination are monitored using an optical system capable of detectingand/or monitoring interactions between reactants at the single-moleculelevel. Such an optical system achieves these functions by firstgenerating and transmitting an incident wavelength to the reactants,followed by collecting and analyzing the optical signals from thereactants. Such systems typically employ an optical train that directssignals from the reactions to a detector, and in certain embodiments inwhich a plurality of reactions is disposed on a solid surface, suchsystems typically direct signals from the solid surface (e.g., array ofconfinements) onto different locations of an array-based detector tosimultaneously detect multiple different optical signals from each ofmultiple different reactions. In particular, the optical trainstypically include optical gratings or wedge prisms to simultaneouslydirect and separate signals having differing spectral characteristicsfrom each confinement in an array to different locations on an arraybased detector, e.g., a CCD, and may also comprise additional opticaltransmission elements and optical reflection elements.

An optical system applicable for use with the present inventionpreferably comprises at least an excitation source and a photondetector. The excitation source generates and transmits incident lightused to optically excite the reactants in the reaction. Depending on theintended application, the source of the incident light can be a laser,laser diode, a light-emitting diode (LED), a ultra-violet light bulb,and/or a white light source. Further, the excitation light may beevanescent light, e.g., as in total internal reflection microscopy,certain types of waveguides that carry light to a reaction site (see,e.g., U.S. Application Pub. Nos. 20080128627, 20080152281, and200801552280), or zero-mode waveguides, described below. Where desired,more than one source can be employed simultaneously. The use of multiplesources is particularly desirable in applications that employ multipledifferent reagent compounds having differing excitation spectra,consequently allowing detection of more than one fluorescent signal totrack the interactions of more than one or one type of moleculessimultaneously. A wide variety of photon detectors or detector arraysare available in the art. Representative detectors include but are notlimited to optical reader, high-efficiency photon detection system,photodiode (e.g. avalanche photo diodes (APD)), camera, charge coupledevice (CCD), electron-multiplying charge-coupled device (EMCCD),intensified charge coupled device (ICCD), and confocal microscopeequipped with any of the foregoing detectors. For example, in someembodiments an optical train includes a fluorescence microscope capableof resolving fluorescent signals from individual sequencing complexes.Where desired, the subject arrays of optical confinements containvarious alignment aides or keys to facilitate a proper spatial placementof the optical confinement and the excitation sources, the photondetectors, or the optical train as described below.

The subject optical system may also include an optical train whosefunction can be manifold and may comprise one or more opticaltransmission or reflection elements. Such optical trains preferablyencompass a variety of optical devices that channel light from onelocation to another in either an altered or unaltered state. First, theoptical train collects and/or directs the incident wavelength to thereaction site (e.g., optical confinement). Second, it transmits and/ordirects the optical signals emitted from the reactants to the photondetector. Third, it may select and/or modify the optical properties ofthe incident wavelengths or the emitted wavelengths from the reactants.In certain embodiments, the optical train controls an on/off cycle ofthe illumination source to provide illuminated and non-illuminatedperiods to one or more illuminated reaction sites. Illustrative examplesof such optical transmission or reflection elements are diffractiongratings, arrayed waveguide gratings (AWG), optic fibers, opticalswitches, mirrors (including dichroic mirrors), lenses (includingmicrolenses, nanolenses, objective lenses, imaging lenses, and thelike), collimators, optical attenuators, filters (e.g., polarization ordichroic filters), prisms, wavelength filters (low-pass, band-pass, orhigh-pass), planar waveguides, wave-plates, delay lines, and any otherdevices that guide the transmission of light through proper refractiveindices and geometries. One example of a particularly preferred opticaltrain is described in U.S. Patent Pub. No. 20070036511, filed Aug. 11,2005, and incorporated by reference herein in its entirety for allpurposes.

In a preferred embodiment, a reaction site (e.g., optical confinement)containing a reaction of interest is operatively coupled to a photondetector. The reaction site and the respective detector can be spatiallyaligned (e.g., 1:1 mapping) to permit an efficient collection of opticalsignals from the reactants. In certain preferred embodiments, a reactionsubstrate is disposed upon a translation stage, which is typicallycoupled to appropriate robotics to provide lateral translation of thesubstrate in two dimensions over a fixed optical train. Alternativeembodiments could couple the translation system to the optical train tomove that aspect of the system relative to the substrate. For example, atranslation stage provide a means of removing a reaction substrate (or aportion thereof) out of the path of illumination to create anon-illuminated period for the reaction substrate (or a portionthereof), and returning the substrate at a later time to initiate asubsequent illuminated period. An exemplary embodiment is provided inU.S. Patent Pub. No. 20070161017, filed Dec. 1, 2006.

In particularly preferred aspects, such systems include arrays ofreaction regions, e.g, zero-mode waveguide arrays, that are illuminatedby the system, in order to detect signals (e.g., fluorescent signals)therefrom, that are in conjunction with analytical reactions beingcarried out within each reaction region. Each individual reaction regioncan be operatively coupled to a respective microlens or a nanolens,preferably spatially aligned to optimize the signal collectionefficiency. Alternatively, a combination of an objective lens, aspectral filter set or prism for resolving signals of differentwavelengths, and an imaging lens can be used in an optical train, todirect optical signals from each confinement to an array detector, e.g.,a CCD, and concurrently separate signals from each different confinementinto multiple constituent signal elements, e.g., different wavelengthspectra, that correspond to different reaction events occurring withineach confinement. In preferred embodiments, the setup further comprisesmeans to control illumination of each confinement, and such means may bea feature of the optical system or may be found elsewhere is the system,e.g., as a mask positioned over an array of confinements. Detaileddescriptions of such optical systems are provided, e.g, in U.S. PatentPub. No. 20060063264, filed Sep. 16, 2005, which is incorporated hereinby reference in its entirety for all purposes.

The systems of the invention also typically include informationprocessors or computers operably coupled to the detection portions ofthe systems, in order to store the signal data obtained from thedetector(s) on a computer readable medium, e.g., hard disk, CD, DVD orother optical medium, flash memory device, or the like. For purposes ofthis aspect of the invention, such operable connection provide for theelectronic transfer of data from the detection system to the processorfor subsequent analysis and conversion. Operable connections may beaccomplished through any of a variety of well known computer networkingor connecting methods, e.g., Firewire®, USB connections, wirelessconnections, WAN or LAN connections, or other connections thatpreferably include high data transfer rates. The computers alsotypically include software that analyzes the raw signal data, identifiessignal pulses that are likely associated with incorporation events, andidentifies bases incorporated during the sequencing reaction, in orderto convert or transform the raw signal data into user interpretablesequence data (See, e.g., Published U.S. Patent Application No.2009-0024331, the full disclosure of which is incorporated herein byreference in its entirety for all purposes).

Exemplary systems are described in detail in, e.g., U.S. patentapplication Ser. No. 11/901,273, filed Sep. 14, 2007 and U.S. patentapplication Ser. No. 12/134,186, filed Jun. 5, 2008, the fulldisclosures of which are incorporated herein by reference in theirentirety for all purposes.

Further, as noted above, the invention provides data processing systemsfor transforming sequence read data into consensus sequence data. Incertain embodiments, the data processing systems include machines forgenerating sequence read data by interrogating a template nucleic acidmolecule. In certain preferred embodiments, the machine generates thesequence read data using a sequencing-by-synthesis technology, asdescribed elsewhere herein, but the machine may generate the sequenceread data using other sequencing technologies known to those of ordinaryskill in the art, e.g., pyrosequencing, ligation-mediated sequencing,Sanger sequencing, capillary electrophoretic sequencing, etc. Suchmachines and methods for using them are available to the ordinarypractioner.

The sequence read data generated is representative of the nucleotidesequence of the template nucleic acid molecule only to the extent that agiven sequencing technology is able to generate such data, and so maynot be identical to the actual sequence of the template nucleic acidmolecule. For example, it may contain a deletion or a different base ata given position as compared to the actual sequence of the template,e.g., when a base call is missed or incorrect, respectively. As such, itis beneficial to generate redundant sequence read data, and the methodsdescribed herein provide manipulations and computations that transformredundant sequence read data into consensus sequence data that isgenerally more representative of the actual sequence of the templatenucleic acid molecule than sequence read data from a single read of asingle template nucleic acid molecule. Redundant sequence read datacomprises multiple reads, each of which includes at least a portion ofsequence read that overlaps with at least a portion of at least oneother of the multiple reads. As such, the multiple reads need not alloverlap with one another, and a first subset may overlap for a differentportion of the template nucleic acid sequence than does a second subset.Such redundant sequence read data can be generated by various methods,including repeated sequencing of a single nucleic acid template,sequencing of multiple identical nucleic acid templates, or acombination thereof.

In another aspect, the data processing systems can include software andalgorithm implementations provided herein, e.g. those configured totransform redundant sequence read data into consensus sequence data,which, as noted above, is generally more representative of the actualsequence of the template nucleic acid molecule than sequence read datafrom a single read of a single template nucleic acid molecule. Further,the transformation of the redundant sequence read data into consensussequence data identifies and negates some or all of the single-readvariation between the multiple reads in the redundant sequence readdata. As such, the transformation provides a representation of theactual nucleotide sequence of the nucleic acid template from whichredundant sequence read data is generated that is more accurate than arepresentation based on a single read.

The software and algorithm implementations provided herein arepreferably machine-implemented methods, e.g., carried out on a machinecomprising computer-readable medium configured to carry out variousaspects of the methods herein. For example, the computer-readable mediumpreferably comprises at least one or more of the following: a) a userinterface; b) memory for storing redundant sequence read data; c) memorystoring software-implemented instructions for carrying out thealgorithms for transforming redundant sequence read data into consensussequence data; d) a processor for executing the instructions; e)software for recording the results of the transformation into memory;and f) memory for recordation and storage of the resulting consensussequence read data. In preferred embodiments, the user interface is usedby the practitioner to manage various aspects of the machine, e.g., todirect the machine to carry out the various steps in the transformationof redundant sequence read data into consensus sequence data,recordation of the results of the transformation, and management of theconsensus sequence data stored in memory.

As such, in preferred embodiments, the methods further comprise atransformation of the computer-readable medium by recordation of theredundant sequence read data and/or the consensus sequence datagenerated by the methods. Further, the computer-readable medium maycomprise software for providing a graphical representation of theredundant sequence read data and/or the consensus sequence read data,and the graphical representation may be provided, e.g., in soft-copy(e.g., on an electronic display) and/or hard-copy (e.g., on a print-out)form.

The invention also provides a computer program product comprising acomputer-readable medium having a computer-readable program codeembodied therein, the computer readable program code adapted toimplement one or more of the methods described herein, and optionallyalso providing storage for the results of the methods of the invention.In certain preferred embodiments, the computer program product comprisesthe computer-readable medium described above.

In another aspect, the invention provides data processing systems fortransforming sequence read data from one or more sequencing reactionsinto consensus sequence data representative of an actual sequence of oneor more template nucleic acids analyzed in the one or more sequencingreactions. Such data processing systems typically comprise a computerprocessor for processing the sequence read data according to the stepsand methods described herein, and computer usable medium for storage ofthe initial sequence read data and/or the results of one or more stepsof the transformation (e.g., the consensus sequence data), such as thecomputer-readable medium described above.

As shown in FIG. 9, the system 900 includes a substrate 902 thatincludes a plurality of discrete sources of chromophore emissionsignals, e.g., an array of zero-mode waveguides 904. An excitationillumination source, e.g., laser 906, is provided in the system and ispositioned to direct excitation radiation at the various signal sources.This is typically done by directing excitation radiation at or throughappropriate optical components, e.g., dichroic 108 and objective lens910, that direct the excitation radiation at the substrate 902, andparticularly the signal sources 904. Emitted signals from the sources904 are then collected by the optical components, e.g., objective 910,and passed through additional optical elements, e.g., dichroic 908,prism 912 and lens 914, until they are directed to and impinge upon anoptical detection system, e.g., detector array 916. The signals are thendetected by detector array 916, and the data from that detection istransmitted to an appropriate data processing system, e.g., computer918, where the data is subjected to interpretation, analysis, andultimately presented in a user ready format, e.g., on display 920, orprintout 922, from printer 924. As will be appreciated, a variety ofmodifications may be made to such systems, including, for example, theuse of multiplexing components to direct multiple discrete beams atdifferent locations on the substrate, the use of spatial filtercomponents, such as confocal masks, to filter out-of focus components,beam shaping elements to modify the spot configuration incident upon thesubstrates, and the like (See, e.g., Published U.S. Patent ApplicationNos. 2007/0036511 and 2007/095119, and U.S. patent application Ser. No.11/901,273, all of which are incorporated herein by reference in theirentireties for all purposes.)

VI. Exemplary Applications

The methods and compositions of the invention are useful in a broadrange of analytical reactions in which one or more aspects of adetection method are detrimental to one or more aspects of theanalytical reaction, such as rate, duration, fidelity, processivity, andthe like. In such cases, intermittent detection at least partiallymitigates the detrimental effect while allowing collection of data fromstages of the analytical reaction that were previously uncollectable. Asnoted above, illuminated reactions are one example of analyticalreactions that benefit from the compositions and methods describedherein, particularly those using photoluminescent or fluorescentreagents, and particularly such reactions where one or more of thereaction components that are susceptible to photo-induced damage arepresent at relatively low levels. One exemplary application of themethods and compositions described herein is in single moleculeanalytical reactions, where the reaction of a single molecule (or verylimited number of molecules) is observed in the analysis, such asobservation of the action of a single enzyme molecule. In anotheraspect, the present invention is directed to illuminated reactions forsingle molecule analysis, including sequencing of nucleic acids byobserving incorporation of nucleotides into a nascent nucleic acidsequence during template-directed polymerase-based synthesis. Suchmethods, generally referred to as “sequencing-by-incorporation” or“sequencing-by-synthesis,” involve the observation of the addition ofnucleotides or nucleotide analogs in a template-dependent fashion inorder to determine the sequence of the template strand. See, e.g., U.S.Pat. Nos. 6,780,591, 7,037,687, 7,344,865, 7,302,146. Processes forperforming this detection include the use of fluorescently labelednucleotide analogs within a confined observation region, e.g., within ananoscale well and/or tethered, either directly or indirectly to asurface. By using excitation illumination (i.e., illumination of anappropriate wavelength to excite the fluorescent label and induce adetectable signal), the fluorescently labeled bases can be detected asthey are incorporated into the nascent strand, thus identifying thenature of the incorporated base, and as a result, the complementary basein the template strand.

In particular aspects, when an analysis relies upon a small populationof reagent molecules, damage to any significant fraction of thatpopulation will have a substantial impact on the analysis beingperformed. For example, prolonged interrogation of a limited populationof reagents, e.g., fluorescent analogs and enzymes, can lead tophoto-induced damage of the various reagents to the point ofsubstantially impacting the activity or functionality of the enzyme. Ithas been shown that prolonged illumination of DNA polymerases involvedin synthesis using fluorescent nucleotide analogs results in a dramaticdecrease in the enzyme's ability to synthesize DNA, often measured as areduction in processivity. Without being bound to any theory ofoperation, it is believed that in some cases a photo-induced damageevent affects the catalytic region of the enzyme thus affecting eitherthe ability of the enzyme to remain complexed with the template, or itsability to continue synthesis. In general, the methods, devices, andsystems of the present invention can increase performance and/orselectively monitor one or more stages of an illuminated reaction bysubjecting the reaction to intermittent illumination.

One particularly preferred aspect of the invention is in conjunctionwith the sequencing by incorporation of nucleic acids within an opticalconfinement, such as a zero-mode waveguide. Such reactions involveobservation of an extremely small reaction volume in which one or only afew polymerase enzymes and their fluorescent substrates may be present.Zero-mode waveguides, and their use in sequencing applications aregenerally described in U.S. Pat. Nos. 6,917,726 and 7,033,764, andpreferred methods of sequencing by incorporation are generally describedin Published U.S. Patent Application No. 2003-0044781, the fulldisclosures of which are incorporated herein by reference in theirentireties for all purposes, and in particular for their teachingsregarding such sequencing applications and methods. Briefly, arrays ofzero-mode waveguides (“ZMWs”), configured in accordance with the presentinvention may be employed as optical confinements for single moleculeDNA sequence determination. In particular, as noted above, these ZMWsprovide extremely small observation volumes at or near the transparentsubstrate surface, also termed the “base” of the ZMW. A nucleic acidsynthesis complex, e.g., template sequence, polymerase, and primer,which is immobilized at the base of the ZMW, may then be specificallyobserved during synthesis to monitor incorporation of nucleotides in atemplate dependent fashion, and thus provide the identity and sequencesof nucleotides in the template strand. This identification is typicallyaccomplished by providing detectable label groups, such as fluorescentlabeling molecules, on the nucleotides. In some instances, the labelednucleotides terminate primer extension, allowing a “one base at a time”interrogation of the complex. If, upon exposure to a given labeled base,a base is incorporated, its representative fluorescent signal may bedetected at the base of the ZMW. If no signal is detected, then the basewas not incorporated and the complex is interrogated with each of theother bases, in turn. Once a base is incorporated, the labeling group isremoved, e.g., through the use of a photocleavable linking group, andwhere the label was not the terminating group, a terminator, upon the 3′end of the incorporated nucleotide, may be removed prior to subsequentinterrogation. In other more preferred embodiments, the incorporation ofa labeled nucleotide does not terminate primer extension and theprocessive incorporation of multiple labeled nucleotides can bemonitored in real time by detecting a series of fluorescent signals atthe base of the ZMW. In some such embodiments, the label is naturallyreleased upon incorporation of the labeled nucleotides by thepolymerase, and so need not be released by alternative means, e.g., aphotocleavage event. As such, a processive sequencing reaction cancomprise a polymerase enzyme repetitively incorporating multiplenucleotides or nucleotide analogs, as long as such are available to thepolymerase within the reaction mixture, e.g., without stalling on thetemplate nucleic acid. (Such a processive polymerization reaction can beprevented by incorporation of nucleotides or nucleotide analogs thatcontain groups that block additional incorporation events, e.g., certainlabeling groups or other chemical modifications.)

In accordance with the present invention, sequencing reactions may becarried out by only interrogating a reaction mixture, e.g., detectingfluorescent emission for one or more illuminated periods beforeexcessive photo-induced damage has occurred. In general, the methodsdescribed herein are implemented in a manner sufficient to providebeneficial impact, e.g., reduced photo-induced damage and/or extensionof the photo-induced damage threshold period, but are not implemented insuch a manner to interfere with the reaction of interest, e.g., asequencing reaction. The present invention also contemplates alternativemethods of and compositions for mitigating the impact of photo-induceddamage on a reaction, as described above and in, e.g., U.S. Ser. No.61/116,048, filed Nov. 19, 2008. Such alternative methods and compoundscan be used in combination with the compositions and methods providedherein to further alleviate the effects of species that can be generatedduring an illuminated reaction.

Another method of mitigating the impact of photo-induced damage on theresults of a given reaction provides for the elimination of potentiallydamaging oxygen species using means other than the use of thephoto-induced damage mitigating agents described above. In one example,dissolved oxygen species may be flushed out of aqueous systems byproviding the reaction system under different gas environments, such asby exposing an aqueous reaction to neutral gas environments, such asargon, nitrogen, helium, xenon, or the like, to prevent dissolution ofexcess oxygen in the reaction mixture. By reducing the initial oxygenload of the system, it has been observed that photo-induced damageeffects, e.g., on polymerase mediated DNA synthesis, is markedlyreduced. In particularly preferred aspects, the system is exposed to axenon atmosphere. In particular, since xenon can be induced to form adipole, it operates as a triplet-state quencher in addition tosupplanting oxygen in the aqueous system. (See, e.g., Vierstra and Poff,Plant Physiol. 1981 May; 67(5): 996-998) As such, xenon would also becategorized as a quencher, as set forth above.

Although described in terms of zero-mode waveguides, it will beappreciated that a variety of selective illumination strategies may beemployed to selectively interrogate different regions of a solid supportover time, e.g., so as to only damage molecules within certain selectedregions of a substrate while not damaging molecules in other selectedregions of the substrate. In certain embodiments, such methods caninvolve using a directed light source (e.g., a laser) to illuminate onlyselected regions; changing the illumination angle of the light source;or refocusing the illumination, e.g., by passing the illuminationthrough an optical train that alters the shape of the incident light onthe solid support. These and further examples of alternative methods ofmitigating photo-induced damage which can be used in combination withmethods and systems of the invention described herein are provided inU.S. Patent Pub. No. 20070036511, filed Aug. 11, 2005; U.S. Pat. No.6,881,312; U.S. Ser. No. 61/116,048, filed Nov. 19, 2008; and U.S.Patent Pub. No. 20070161017, filed Dec. 1, 2006, all of which areincorporated herein by reference in their entireties for all purposes,and in particular for disclosure related to these methods of mitigatingphoto-induced damage.

As noted above, using templates that allow repeated sequencing (e.g.,circular templates, SMRTbell™ templates, etc.) in a single reaction canincrease the percent of a nucleic acid template for which nucleotidesequence data is generated and/or increase the fold-coverage of thesequence reads for one or more regions of interest in the template,thereby providing more complete data for further analysis, e.g.,construction of sequence scaffolds and/or consensus sequences for thenucleic acid template. For example, in certain preferred embodiments,templates sequenced by the methods described herein are templatescomprising a double-stranded segment, e.g., greater than 75%, or evengreater than 90% of the target segment will be double-stranded orotherwise internally complementary. Such templates may, for example,comprise a double-stranded portion comprised of two complementarysequences and two single-stranded linking portions (e.g., oligos or“hairpins”) joining the 3′ end of each strand of the double-strandedregion to the 5′ end of the other strand (sometimes referred to as“SMRTbell™” templates). In certain embodiments, double-stranded portionsfor use in such templates are PCR-amplified. Optionally, restrictionsites are incorporated within the PCR primers such that subsequentdigestion of the amplified products with appropriate restriction enzymesgenerates double-stranded portions containing known overhang sequenceson either end, which are then ligated to hairpin adapters containing acomplementary overhang to generate the SMRTbell™ templates.

These template molecules are particularly useful as nucleotide sequencedata generated therefrom comprises both sense and antisense nucleotidesequences for the double-stranded portion, and the circular conformationof the template enables repeated sequencing (e.g., using a polymerasecapable of strand-displacement) provides duplicative or redundantsequence information. Restated, a sequence process may progress aroundthe completely contiguous sequence repeatedly obtaining sequence datafor each segment from the complementary sequences, as well as sequencedata within each segment, by repeatedly sequencing that segment.Iterative illumination is useful in such sequencing applications, e.g.,to focus nucleotide sequence data collection on stages of the sequencingreaction most of interest, such as the stages during which nucleotidesequence data is being generated from a strand of the (previously)double-stranded portion. Iterative illumination may also allowadditional “rounds” of sequencing the template by virtue of thereduction in photo-induced damage to reaction components, as describedelsewhere herein, thereby providing more complete and robust nucleotidesequence data for future analysis, e.g., sequence scaffold constructionand/or consensus sequence determination. Further, as described above,the number of base positions separating sequence reads generated inilluminated periods can be estimated based on the temporal length ofintervening non-illuminated periods and the known rate of incorporationduring the reaction and/or by the measured rate of incorporation duringthe illuminated period(s). The known rate of incorporation can be basedon various factors including, but not limited to, sequence contexteffects due to the nucleotide sequence of the template nucleic acid,kinetics of the polymerase used, buffer effects (salt concentration, pH,etc.), and even data being collected from an ongoing reaction. Thesefactors can be used to determine the appropriate timing for theilluminated and non-illuminated periods depending on the experimentalobjectives of the practitioner, whether it be maximizing length or depthof sequence coverage on a given template nucleic acid, or optimizingsequence data collection from particular regions of interest, e.g., fromthe ends of the double-stranded portion of a SMRTbell™ template.

In addition to providing sense and antisense sequence data within asingle template molecule that can be sequenced in one integratedprocess, the presence of the single-stranded linking portions alsoprovides an opportunity to provide a registration sequence that permitsthe identification of when one segment, e.g., the sense strand, iscompleted and the other begins, e.g., the antisense strand. Suchregistration sequences provide a basis for alignment sequence data frommultiple sequence reads from the same template sequences, e.g., the samemolecule, or identical molecules in a template population. Additionalaspects of and uses for registration sequences, e.g., for molecularredundant sequencing, are further described in U.S. Patent PublicationNo. 20090029385, which is incorporated herein by reference in itsentirety for all purposes.

In certain embodiments, such a sequencing process begins by priming thetemplate nucleic acid within one of the linking portions and allowingthe polymerase to proceed along the strand of the double-strandedportion of the template that is immediately downstream of the primedlinking portion when the double-stranded portion is melted or denatured.The sequence process proceeds around the second linking portion andproceeds along the complementary strand of the (now previously)double-stranded portion of the template. Because the template iscircular, this process can continue to provide multiple repeatedsequence reads from the one template. Thus, sequence redundancy comesfrom both the determination of complementary sequences (sense andantisense strands of the double-stranded portion), and the repeatedsequencing of each circular template. The ongoing sequencing reaction issubjected to multiple illuminated and non-illuminated periods togenerate at least two or more sequence reads per pass around thetemplate. The illuminated periods are preferably timed to allowgeneration of nucleotide sequence data for selected regions of thetemplate. For example, it may be beneficial to only generate nucleotidesequence data for the complementary strands of the double-strandedportion, or segments thereof. As will be appreciated, in iterativelysequencing circular templates, strand displacing polymerases, asdiscussed elsewhere herein, are particularly preferred, as they willdisplace the nascent strand with each cycle around the template,allowing continuous sequencing. Other approaches will similarly allowsuch iterative sequencing including, e.g., use of an enzyme having 5′-3′exonuclease activity in the reaction mixture to digest the nascentstrand post-synthesis.

One may optionally employ various means for controlling initiationand/or progression of a sequencing reaction, and such means may includethe addition of specific sequences or other moieties into the templatenucleic acid, such as binding sites, e.g., for primers or proteins.Various methods of incorporating control elements into an analyticalreaction, e.g. by integrating stop or pause points into a template, arediscussed elsewhere herein and are further described in relatedapplication, U.S. application Ser. No. 12/413,258, filed Mar. 27, 2009,which is incorporated herein by reference in its entirety for allpurposes.

In certain embodiments, a reaction stop or pause point may be includedwithin the template sequence, such as a reversibly bound blocking groupat one location on the template, e.g., on the linking portion that wasnot used in priming. By way of example, following initial sequencingfrom the original priming location, e.g., from the single-strandedlinking portion used in priming synthesis through a first portion of thesense strand (e.g., the 3′ end), the data acquisition may be switchedoff and the polymerase allowed to proceed around the template, e.g.,through the remainder of the sense strand to the other linking portion.The incorporation of a synthesis blocking moiety coupled to this linkingportion will allow control of reinitiation of the polymerase activity atthe 3′ end of the antisense strand. One would thereby obtain paired-endsequence data for the overall (previously) double-stranded segment, withsequence data from one end coming from the sense strand and sequencedata from the other end coming from the antisense strand. This templateconstruction and sequencing methodology is particularly useful in thecase of long double-stranded segments, especially given the short readlengths generated by some sequencing technologies.

A variety of synthesis controlling groups may be employed, including,e.g., large photolabile groups coupled to the nucleobase portion of oneor more bases in the single-stranded portion that inhibitpolymerase-mediated replication; strand-binding moieties that preventprocessive synthesis; non-native nucleotides included within the primerand/or template; and the like. The use of strand-binding moietiesincludes, but is not limited to, reversible, specific binding ofparticular proteins to recognition sequences incorporated into thetemplate (or primer bound thereto) for this purpose. In certainembodiments, such control sequences may include binding sites fortranscription factors, e.g., repressor binding regions provided withinthe linking portion(s). For example, the lac repressor recognitionsequence is bound by the lac repressor protein, and this binding hasbeen shown to block replication in a manner reversible by addition ofappropriate initiators, such as isophenylthiogalactoside (IPTG) orallolactose.

In some embodiments, primer recognition sequences and/or additionalcontrol sequences may also be provided for control of initiation and/orprogression of polymerization, e.g., through a hybridized probe orreversibly modified nucleotide, or the like. (See, e.g., U.S. PatentApplication No. 2008-0009007, the full disclosure of which isincorporated herein by reference in its entirety for all purposes.) Suchprobes include but are not limited to probes at which a polymeraseinitiates polymerization, probes containing various types of detectablelabels, molecular beacons, TaqMan® probes, Invader® probes (Third WaveTechnologies, Inc.), or the like, that can be used for various purposes,e.g., to provide indications of the commencement and/or progress ofsynthesis.

An engineered pause point (reversible or irreversible) can include oneor more non-native (non-natural) or fifth bases that do not pair withany of the four native nucleoside polyphosphates in the synthesisreaction, e.g., in the template and/or oligonucleotides probe(s), and/orthat exhibit a distinct kinetic signature during template-dependentsynthesis at such a base. Upon encountering such a base, the polymerasepauses until the complement to the non-natural base is added to thereaction mixture. Likewise, an engineered pause point could include a“damaged” base that causes a stop in replication until repair enzymesare added to the mixture. For example, a template having a pyrimidinedimer would cause the replication complex to pause, and addition of thephotolyase DNA repair enzyme would repair the problem location and allowreplication, and sequencing to continue. In yet further embodiments, acombination of modification enzymes could be used to engineer a set ofmodified bases on a template, e.g., a combination of glycosylases,methylases, nucleases, and the like. (Further information on sequencingtemplate nucleic acids comprising modifications, including detectingkinetic signatures of such modifications during single-moleculesequencing reactions, are provided in U.S. Patent Application Nos.61/201,551, filed Dec. 11, 2008; 61/180,350, filed May 21, 2009; andSer. No. 12/945,767, filed Nov. 12, 2010; and U.S. Patent PublicationNo. 2010/0221716, the disclosures of which are incorporated herein byreference in their entireties for all purposes.)

As noted elsewhere herein, stop or pause points can be engineered intovarious portions of the template, e.g., portions for which thenucleotide sequence is unknown (e.g., a genomic fragment) or known(e.g., an adaptor or linker ligated to the genomic fragment.) Forexample, SMRTbell™ templates are topologically closed, single-strandedmolecules having regions of internal complementarity separated byhairpin or stem-loop linkers, such that hybridization of the regions ofinternal complementarity produces a double-stranded portion within thetemplate. One or both of the linkers can comprise a stop or pause pointto modulate polymerase activity. In some embodiments, these regulatorysequences or sites cause a permanent cessation of nascent strandsynthesis, and in other embodiments the reaction can be reinitiated,e.g., by removing a blocking moiety or adding a missing reactioncomponent. Various types of pause and stop points are described belowand elsewhere herein, and it will be understood that these can be usedindependently or in combination, e.g., in the same template molecule.

In other embodiments, an abasic site is used as a synthesis blockingmoiety or pause point until addition of a non-natural “base,” such as apyrene, which has been shown to “base-pair” with an abasic site duringDNA synthesis. (See, e.g., Matray, et al. (1999) Nature 399(6737):704-8,which is incorporated herein by reference in its entirety for allpurposes.) Where a permanent termination of sequencing is desired, nonon-natural analog is added and the polymerase is permanently blocked atthe abasic site. DNA (or RNA) glycosylases create abasic sites that arequite different from the normal coding bases, A, T, G, and C (and U inRNA). A wide variety of monofunctional and bifunctional DNA glycosylasesthat have specificity for most common DNA or RNA adducts, including5-methylcytosine, are known in the art, with different glycosylasescapable of recognizing different types of modified DNA and/or RNA bases.The molecular structures of many glycosylases have been solved, andbased on structural similarity they are grouped into four superfamilies.The UDG and AAG families contain small, compact glycosylases, whereasthe MutM/Fpg and HhH-GPD families comprise larger enzymes with multipledomains. As an example, four enzymes have been identified in Arabidopsisthaliana in the plant pathway for cytosine demethylation. Additionally,other enzymes are also known to recognize 5-methyl cytosine and removethe methylated base to create an abasic site. Further, various enzymesare known to methylate cytosine in a sequence-specific manner. As such,a combination of a cytosine-methylase and an enzyme that creates anabasic site from a methylated cytosine nucleotide can be used to createone or more abasic sites in a template nucleic acid. The size of therecognition site of the methylase and the base composition of thetemplate determine how frequently methylation occurs, and therefore, thenumber of abasic sites created in a given template nucleic acid,allowing the ordinary practioner to choose a methylase with arecognition site that produces a desired spacing between modifiednucleotides. For example, if the recognition site is three bases long,then on average an abasic site is expected every 64 bases; if therecognition site is four bases long, then on average an abasic site isexpected every 256 bases; if the recognition site is six bases long,then on average an abasic site is expected every 4096 bases; and soforth. Of course, templates with a higher GC content would be expectedto have more frequent abasic site formation, and templates with lower GCcontent would be expected to have less frequent abasic site formation.

Uracil-DNA glycosylases can also be used to introduce abasic sites intoa template nucleic acid comprising deoxyuridine nucleotides. Thisstrategy has the advantage of allowing the practitioner to choose thelocations of the abasic sites within a DNA template since deoxyuridinenucleotides are not generally found in DNA. Various methods of insertingdeoxyuridine nucleotides into a DNA template may be used, and differentmethods will be preferred for different applications. In certainembodiments, one or more site-specific deoxyuracils are incorporatedduring standard phosphoramidite oligonucleotide synthesis. To placeuracils at indeterminate positions in a DNA, replacing a portion of thedeoxythymidine triphosphate with deoxyuridine triphosphate will resultin an amplimer with random U sites in place of T sites after polymerasechain reaction. In other embodiments, deoxyuridine nucleotides areengineered into the template, e.g., by ligation of a synthetic linker oradaptor comprising one or more deoxyuridine nucleotides to a nucleicacid sequence to be sequenced. In certain preferred embodiments,deoxyuridine nucleotides are incorporated into the linker portions of aSMRTbell™ template.

To subsequently introduce abasic sites prior to sequencing, thedeoxyuridine nucleotide-containing template is subjected to treatmentwith uracil-DNA glycosylase, which removes the one or more uracil basesfrom the deoxyuridine nucleotides, thereby generating one or more abasicsites in the template. Alternatively, since the deoxyuridine nucleotidecan be recognized as a template base and paired with deoxyadenosineduring template-dependent nascent strand synthesis, thesynthesis-blocking abasic site can instead be introduced afterinitiation of the sequencing reaction, e.g., at a time chosen by thepractitioner. For example, the reaction can be initiated with adeoxyuridine-containing template, and uracil-DNA glycosylase cansubsequently be added to block the polymerase and halt the reactionafter the reaction has proceeded for a given time. As such, terminationof the reaction is optional rather than required.

While uracil-DNA glycosylase activity is useful for introducing abasicsites into a template as described above, this activity can beproblematic during the preparation of such templates. As such,strategies are typically implemented during preparation and manipulationof uracil-containing DNA, e.g., using molecular biology enzymes, toavoid uracil-DNA glycosylase activity, in particular, due to the E. coliUDG enzyme. Since a majority of standard molecular biology enzymes areoverexpressed and subsequently purified from an E. coli host, UDGactivity can be a contaminating activity that is often not monitored bythe enzyme manufacturer's quality control procedures. To mitigatecontaminating UDG activity, a commercially available UDG inhibitor, alsoknown as uracil glycosylase inhibitor or UGI (e.g., from New EnglandBiolabs, Ipswich, Mass.) can be included in molecular biology reactions.This is a small protein inhibitor from the B. subtilis bacteriophasePBS1 that binds reversibly to E. coli UDG to inhibit its catalyticactivity. UGI is also capable of dissociating UDG from a DNA molecule.Alternatively, UDG activity can be inhibited without exogenous proteinusing a chemical inhibitor of the enzyme, such as an oligonucleotidecontaining a 1-aza-deoxyribose base, a transition state analog for theUDG enzyme. This and other cationic nitrogenous sugars have been usedfor mechanistic studies of UDG activity and show potent inhibitionactivity. (See, e.g., Jiang et al. Biochemistry, 2002, 41 (22), pp7116-7124.)

In certain applications, UDG activity needs to be inhibited temporarily,and subsequently enabled to remove create an abasic site as describedabove. In some embodiments, a DNA purification that removes proteins isemployed, e.g., including a phenol-chloroform extraction with subsequentethanol precipitation, a silica-based column approach (e.g., QiaQuickcolumns from Qiagen and similar products), and/or a PEG/sodium chlorideprecipitation (e.g., AMPure beads from Beckman Coulter). Alternativelyor additionally, a commercially-available UDG enzyme that is notinhibited by UGI is added when abasic site formation is desired. Forexample, the A. fulgidus UDG is from a thermophilic organism and cannotbe inhibited by the same bacteriophage protein as is the E. coli UDGenzyme. In certain preferred embodiments, UDG-inhibition is employedduring template preparation, and inhibition-resistant UDG activity isadded at a subsequent time to trigger the creation of abasic sites atdeoxyuridine nucleotides, e.g., immediately prior to or during anongoing reaction.

In some preferred embodiments, one or more abasic sites are engineeredinto a linker or adapter sequence within a sequencing template molecule.Abasic sugar residues serve as efficient terminators of polymerizationfor many polymerases, e.g., Φ29. 1′,2′-dideoxyribose is the most commonsynthetic “abasic site”. In other embodiments, a synthetic linker isincorporated into a linker or adaptor. For example, an internal spacer(e.g., Spacer 3 from Biosearch Technologies, Inc.) or other carbon-basedlinker can be used in lieu of a sugar-base nucleotide. Similar to anabasic nucleotide, the polymerase will be blocked upon encounteringthese moieties in the template nucleic acid.

In certain embodiments, synthesis blocking moieties are nicks in thetemplate nucleic acid. Nicking enzymes (e.g., nicking endonucleases) areknown in the art and can be used to specifically nick the template priorto or during a template-directed sequencing reaction. The use ofsite-specific nicking endonucleases allows the practitioner toincorporate a recognition sequence at a particular location within thetemplate nucleic acid, and such nicking endonucleases are commerciallyavailable, e.g., from New England Biolabs, Inc. For example, a linker oradapter can be synthesized with a nicking endonuclease recognitionsequence, ligated to a nucleic acid molecule to be sequenced, and can bespecifically nicked either before or during a subsequent sequencingreaction. Nicks can also be introduced by ligating duplex segments thatlack either a terminal 3′-hydroxy (e.g., have a dideoxynucleotide at the3′-terminius) and/or 5′-phosphate group on one strand. The ligationresults in covalent linkage of the phosphodiester backbone on onestrand, but not on the other, which is therefore effectively “nicked.”In certain embodiments, a SMRTbell™ template is constructed using aduplex (or “insert”) nucleic acid molecule lacking a 5′-phosphate groupat one or both termini. Upon ligation of the hairpin or stem-loopadaptors at each end, nicks are created at one or both ligation site(s),depending on whether the duplex lacked a 5′-phosphate at one or bothends, respectively. In other embodiments, a SMRTbell™ template isconstructed using one or two stem-loop adaptors lacking a 3′-hydroxygroup at the terminus (e.g., comprising a 2′,3′-dideoxynucleotide ratherthan a 2′-deoxynucleotide). Upon ligation of one or two stem-loopadaptors lacking a 3′-hydroxy group, one or two nicks are created at theligation site(s), depending on whether one or two adaptors lacked the3′-hydroxy group, respectively. In both cases, a nick is created in thetemplate nucleic acid, and a primer bound to one of the adaptorsprovides an initiation site for the polymerase, which will process thetemplate until encountering a nick, at which point the polymerase willterminate the reaction by dissociation from the template. Regardless ofhow a nick is created, the position of a nick relative to the initiationsite for the polymerase determines how much of the template will besequenced. For example, FIG. 19A provides an illustrative example of anembodiment in which a nick is present on a first strand of a duplexportion at a position distal to the adaptor containing the primerbinding site. The first strand is processed by a polymerase, but thecomplementary strand is not processed because the polymerase dissociatesat the nick site. An alternative embodiment is shown in FIG. 19B, inwhich a nick is present on the strand complementary to the first strandat a position proximal to the adaptor containing the primer bindingsite. In this case both the first and complementary strands, as well asthe adaptor not containing the primer binding site, are processed by thepolymerase prior to dissociation. The position of the primer bindingsite also determines how much of the template is processed by thepolymerase. FIG. 19C provides a template having a primer binding site ata position from which a polymerase would process a significant portionof the adaptor prior to entering the duplex portion. An additionaladvantage to using a 3′-dideoxynucleotide at a nick is that it preventsthe use of the nick as a polymerse initiation site, since strandextension requires a 3-hydroxy group. As such, the resulting nick wouldnot compete with a primer site for initiation of nascent strandsynthesis by the polymerase. Having a single, known site of initiationon a template molecule is beneficial, e.g., for subsequent mapping of aread generated in such a reaction. In certain preferred embodiments, anick site both lacks a 5′-phosphate group and comprises a3′-dideoxynucleotide.

In certain preferred embodiments, modification and base excision isperformed prior to introduction of a template nucleic acid to a reactionsite, e.g., a zero-mode waveguide. As noted above, the choice ofrecognition site for the methylase depends on how far apart thepractitioner wishes point of synthesis initiation to be on the template.For example, after initiating the template-dependent sequencingreaction, the sequence of nucleotide incorporations into the nascentstrand is monitored for a desired sequence read, which may extend fromthe initiation point to the pause point, or may end before thepolymerase reaches the pause point. In some preferred embodiments, asdescribed elsewhere herein, the monitoring is suspended by modifying orremoving an illumination source, e.g., by moving the illumination sourceor a substrate comprising the reaction site. Synthesis of the nascentstrand will continue until the pause site is reached, whether or not thereaction is being actively monitored. When the reaction is to bereinitiated, reaction components are added that allow bypass, e.g.,pyrene, polymerase, etc., and these can be subsequently removed (e.g.,by buffer exchange) to allow additional pauses at other pause sites onthe template.

In certain embodiments using pyrosequencing-based technologies (e.g., asdeveloped by 454 Life Sciences), abasic sites can be introduced into aset of amplified template nucleic acids and synthesis initiated. Sinceall templates in the set are identical, they will comprise the samenumber of abasic sites in the same positions. During the course of thesynthesis reaction, the synchronous incorporation of nucleotides intothe nascent strands is monitored until either an abasic site is reached(at which point the synthesis is paused) or until the incorporationbecomes asynchronous (which increases the background noise and decreasesreliability of the sequence read). In the latter case, the practionermay opt to speed up the reaction, e.g., by adding all nucleotides at onetime, to extend all nascent strands to the first abasic site in thetemplates. When synthesis is to be reinitiated, reaction components areadded that allow bypass of the abasic site, e.g. one or more pyrenes. Awash step may be performed to remove nucleotides and/or polymerases fromthe reaction sites prior to such addition. Further, in some cases, adifferent polymerase may be used for pyrene incorporation as is used forsequencing-by-synthesis reactions. In certain preferred embodiments, thereaction mixture comprising the pyrene for abasic site bypass allowsreadthrough of the abasic site, but no further on the template.Subsequent addition of sequencing reaction mixture allows thesequencing-by-synthesis reaction to recommence and incorporation ofnucleotides into the nascent strand to be monitored. Alternatively oradditionally, the practioner need not wait until an abasic site isreached to suspend detection and, optionally, speed up the reaction tobring all nascent strands to a given abasic site, but can choose to dothis before a reaction has become asynchronous, e.g., after desiredsequence data has been collected for a particular region of interest ina template nucleic acid.

In certain embodiments using ligation-based technologies (e.g., theSOLiD™ System developed by Life Technologies), a pause site can beengineered by using an oligonucleotide that cannot participate in theligation reaction and that is complementary to a desired location on theset of identical template nucleic acids, e.g., on a bead. When theserial ligation reaction hits the position recognized by thispolynucleotide, the reaction cannot proceed and any reactions that havebecome asynchronous will “catch up.” The user can then unblock the oligo(e.g., using chemical treatment or photo-cleavage) and reinitiate thesequencing reaction.

In some cases, it may be desirable to provide endonuclease recognitionsites within the template nucleic acid. For example, inclusion of suchsites within a circular template can allow for a mechanism to releasethe template from a synthesis reaction, i.e., by linearizing it, andallowing the polymerase to run off the linear template, and/or to exposethe template to exonuclease activity, and thus terminate synthesisthrough removal of the template. Such sites could additionally beexploited as control sequences by providing specific binding locationsfor endonucleases engineered to lack cleavage activity, but retainsequence specific binding, and could therefore be used to blockprogression of the polymerase enzyme on a template nucleic acid.

In some cases, nicking sites, e.g., sites recognized by nickingendonucleases, may be included within a portion of the templatemolecule, and particularly within a double-stranded portion of thetemplate, e.g., in a double-stranded segment of a SMRT Bell™ or in thestem portion of an exogenous hairpin structure. Such nicking sitesprovide one or more breaks in one strand of a double-stranded sequenceand can thereby provide one or more priming locations for, e.g., astrand-displacing polymerase enzyme. A variety of nicking enzymes andtheir recognition sequences are known in the art, with such enzymesbeing generally commercially available, e.g., from New England Biolabs.

In certain embodiments, methods for intermittent detection describedherein are useful in “paired-end” sequencing applications in whichsequence information is generated from two ends of a template nucleicacid but not for at least a portion of the intervening portion of thetemplate. Typically, paired-end sequencing applications provide sequencedata for only the two ends of a nucleic acid template, but the presentinvention also allows generation of additional sequence reads that arenoncontiguous with the sequence reads from the ends of the template. Incertain preferred embodiments, a duplex fragment (e.g., genomicfragment) is ligated to a single-stranded linker that connects the 3′end of the sense strand to the 5′ end of the antisense strand, or thatconnects the 5′ end of the sense strand to the 3′ end of the antisensestrand. In either orientation, separation of the two strands of theduplex fragment results in a single-stranded linear template nucleicacid that contains the linker in between the sense and antisensestrands. Subsequent sequencing can involve intermittent detection thatgenerates sequence reads for only the portions of the sense andantisense strands that are of interest, e.g., one or both of the ends.In certain embodiments, both sense and antisense strands may besequenced at both ends to provide redundancy in the sequence data.Sequence reads recognized as being from the linker portion of thetemplate (e.g., based on the known linker sequence or specificregistration sequences encoded therein) can be used to orient thealignment of the sequence reads from the sense and antisense portions ofthe template, providing context for determining the sequences of theends of the duplex fragment and subsequent sequence scaffoldconstruction and/or mapping. In certain embodiments, pause or stoppoints may be incorporated into the linker to control the processing ofthe template by the polymerase, and therefore may be used to synchronizethe detection periods to ensure generation of sequence reads fromparticular regions of template. Further, additional detection periodscan be included that are timed to provide sequence reads from portionsof the sense and/or antisense strand that are noncontiguous with the endregions.

In a related embodiment, paired-end sequencing may be accomplished byusing a nucleic acid template that has linkers connecting the sense andantisense strands of a duplex fragment at both ends, such thatseparation of the strands of the duplex fragment provides asingle-stranded circular template that contains a linkers in betweeneach end of the sense and antisense strands of the original duplexfragment. Such a template molecule would allow a strand-displacingpolymerase to proceed around the template multiple times, therebypotentially generating redundant sequence data from both ends of bothstrands of the original duplex fragment. As noted elsewhere herein, suchredundancy is useful for determination of consensus sequences and/orconstruction of sequence scaffolds. As the polymerase enzyme processesthe template, detection periods can be timed (e.g., based on knowledgeof the rate at which the polymerase processes the template, which isdependent not only on the polymerase but also on the sequence of thetemplate itself) to generate nucleotide sequence reads from the regionsof the template corresponding to one or both ends of the sense andantisense strands, and can also include detection periods to generateadditional reads from other, noncontiguous regions of the duplexfragment, as well. Although such timing can be used to determine theappropriate periodicity of the detection periods, at later stages of thereaction (e.g., as the polymerase repeatedly proceeds around thetemplate), the exact location of reinitiation of sequence readgeneration becomes more approximate. Incorporation of pause or stoppoints into one or both linkers to regulate the processing of thetemplate by the polymerase may be used to synchronize the detectionperiods regardless of the total distance travelled by the polymerasearound the template. This strategy more reliably ensures generation ofsequence reads from selected regions of template, e.g, the ends of thesense and antisense portions and, optionally, regions in between andnoncontiguous with the end regions regardless of the number of passes ofthe polymerase around the template nucleic acid, especially in laterstages of the reaction. Further, the known sequence of one or both ofthe linkers can be used to orient sequence reads from the sense andantisense portions for consensus sequence determination and/or mapping.

In some such embodiments, a duplex fragment inserted between two hairpinlinkers may be much larger than desired, increasing the difficulty oflimiting nucleotide sequence read data to particular regions of thefragment. The size of the duplex fragment ligated to the two hairpinlinkers can be selectively reduced to retain the regions attached to thelinkers and to lose a central portion of the duplex fragment. Oneparticularly preferred strategy, illustrated in FIG. 18, compriseshairpin linkers (1802, 1804) having a regions of cross-complementarity(1806, 1808), such that the two linkers 1802 and 1804 can anneal to eachother in a manner that does not interfere with ligation to a duplexfragment 1810. Duplex fragment 1810 comprises ends 1812 and 1814, aswell as a long central region 1816, which is not shown but is understoodto be between the two curvy lines. Once end 1812 is ligated to linker1802 and end 1814 is ligated to linker 1804, the construct is subjectedto fragmentation, which removes the central region 1816 of the duplexfragment 1810, producing construct 1818 having ends 1820 and 1822. Afterfragmentation, the ends of the portions of the duplex fragment stillassociated with the annealed linker pair (ends 1820 and 1822) areligated together to produce construct 1824, which can then be treated(e.g., with heat, gentle denaturation, primer invasion, changing saltconcentration, etc.) to separate cross-complementary regions 1806 and1808 from one another, e.g., to generate a circular single-strandednucleic acid molecule. Alternatively, the separation may occur duringthe course of the subsequent reaction, e.g., by polymerase-mediatedstrand displacement. Yet further, where the hybridizedcross-complementary regions are long enough to undergo a complete DNAturn, an additional reaction component (e.g., helicase, topoisomerase,polymerase, etc.) may be needed to unwind the duplex and allowseparation. As such, the resulting “mate-pair” construct has only theends of the original duplex fragment ligated together and capped withadaptors that link the 5′ end of each strand of the duplex with the 3′end of the other strand of the duplex, and denaturation of the duplexproduces a closed, single-stranded circular construct.

Fragmentation of the duplex fragment can be performed by a variety ofknown methods. For example, fragmentation can be performed enzymatically(e.g., using restriction enzymes or other nucleases) or mechanically, byshearing or sonication. The type of fragmentation chosen will determinevarious characteristics of the resulting construct, e.g., how large acentral region is removed and the types of ends remaining (e.g., blunt,5′ overhang, 3′ overhang, random, identical on both ends, etc.).Optionally, the ends can be modified after fragmentation to facilitatethe subsequent ligation step. Although not shown in FIG. 18, it isexpected that the ligation of the duplex fragment to the hybridizedlinkers will be a two-step process, with one end being ligated first andunimolecular kinetics favoring ligation of the second end to the secondlinker. The cross-complementary regions of the linkers can be designedto produce varying levels of complementarity, and therefore varyingstrengths of the hybridization. For example, a longer or higher GCcontent in a cross-complementary region lends a higher stability to thelinker:linker interaction, but separation of the hybridized linkersrequires a more severe treatment, e.g., higher temperature, morestringent conditions, etc. As such the cross-complementary regionsshould be engineered to produce a stable linker:linker interaction thatis disruptable under conditions that are not destructive to the overallconstruct. Further the linkers can vary in regions apart from thecross-complementary regions For example, one linker can have a primerbinding site that the other lacks, which would provide a singlepolymerase initiation site in the final construct. Other sequencecharacteristics described herein (e.g., pause sites, registrationssequences, etc.) can also be included in one or both linker regions. Iftopological constraints limit the subsequent processing of the resultingcontruct, e.g., during template-directed nascent strand synthesis, thesecan be addressed by addition of a reaction component (e.g., a helicaseor topoisomerase) to resolve the topological constraint. As such, themethods can be used to add asymmetric linkers to duplex polynucleotides,whether or not the duplex is to be selectively reduced in size, or not,as long as the asymmetric linkers can cross-hybridize to one another.

Although in preferred embodiments, the two linkers to be ligated to asingle duplex fragment are hybridized to one another prior to ligation,in some embodiments they are instead hybridized after the initialligation reaction, and where topological constraints inhibit such apost-ligation hybridization a reaction component (e.g., topoisomerase)may be included to relieve such constraints. In certain embodiments, thehybridized linkers are separated prior to addition of reactioncomponents for a subsequent reaction, and in other embodiments thehybridized linkers are not separated until after the addition ofreaction components for a subsequent reaction. For example, a polymeraseenzyme may bind to a primer annealed to a linker before or afterseparation of the linker from a second linker. In fact, it may bebeneficial in some embodiments to postpone separation of the linkers,e.g., where compaction of the nucleic acid construct is beneficial, suchas when the construct must be loaded into a confinement of some kind,e.g., a nanowell, optical confinement, etc.

In some embodiments, the methods further include separation of singlelinker constructs from hybridized linker pair constructs. This can beaccomplished by an exonuclease treatment after ligation of the duplexfragment to the linkers, which would degrade any constructs having anunannealed end. Alternatively, it may be desirable to remove the singlelinkers prior to ligation, for example using a size separationmethodology or by allowing them to bind to oligonucleotides that arecomplementary to the cross-complementary regions and bound to a columnor magnetic beads. (The cross-complementary regions of the hybridizedlinker pairs will not be available for binding to the oligonucleotides.Other methods known in the art can also be used to separate singlelinkers from hybridized linker pairs.

Interestingly, the use of the sense/antisense nucleic acid templatesdescribed above would represent a unidirectional processing of atemplate to provide paired-end sequence data, as opposed to the moretraditional bi-directional processing of a linear template molecule.Further, unlike traditional approaches, these methods for paired-endsequencing involve processing, chemically or otherwise, of not just theregions at the ends, but also regions in between the ends, and in someembodiments comprising processing of the entire template. For example, apolymerase incorporates nucleotides into a nascent strand for eachposition of the template (thereby “processing” each position of thetemplate), yet the sequencing data generated is limited to specificregions of the template that are of particular interest to thepractitioner, such as the end regions. As such, in certain embodimentsthe duplex fragment is not further reduced in size after ligation to alinker pair, and the entire duplex fragment is processed by thepolymerase.

In certain embodiments, methods for intermittent detection describedherein are useful in analysis systems that employ nanopores. A nanoporeis a small pore in an electrically insulating membrane that can be usedfor single molecule detection. In general, a nanopore functions as aCoulter counter for much smaller particles, and can take various forms,e.g., a protein channel in a lipid bilayer or a pore in a solid-statemembrane. The detection principal is based on monitoring the ioniccurrent of an electrolyte solution passing through the nanopore as avoltage is applied across the membrane. For example, passage of apolynucleotide molecule (e.g., DNA, RNA, etc.) through a nanopore causeschanges in the magnitude of the current through the nanopore, with eachnucleotide obstructing the nanopore to a different, characteristicdegree. As such, the pattern of variations in the current passingthrough the nanopore as the polynucleotide is drawn through may bemonitored and analyzed to determine the nucleotide sequence of thepolynucleotide. A polynucleotide may be drawn through the nanopore byvarious means, e.g., by electrophoresis, or using enzyme chaperones toguide the polynucleotide through the nanopore. For additional discussionof methods of fabrication and use of nanopores, see, e.g., U.S. Pat. No.5,795,782; Kasianowicz, J. J., et al. (1996) Proc Natl Acad Sci USA93(24):13770-3; Ashkenas, N., et al. (2005) Angew Chem Int Ed Engl44(9):1401-4; Winters-Hilt, S., et al. (2003) Biophys J 84:967-76;Astier, Y., et al. (2006) J Am Chem Soc 128(5):1705-10; Fologea, D., etal. (2005) Nano Lett 5(10):1905-9; Deamer, D. W., et al. (2000) TrendsBiotechnol 18(4):147-51; and Church, G. M. (2006) Scientific American294(1):52, all of which are incorporated by reference herein in theirentireties for all purposes. In some embodiments, intermittent detectionof nucleic acid sequence data from a nanopore may be achieved bymodifying the progress of the polynucleotide through the nanopore sothat progress is sped up during non-detection periods and progress isslowed to allow sequence determination during detection periods. Therate of passage of the polynucleotide through the nanopore may bemodified by various methods, including but not limited to increasing anelectrophoretic field carrying the polynucleotide (e.g., by increasingthe voltage, changing the conductivity of the reaction mixture, and thelike), or changing various reaction conditions to alter the speed atwhich a protein chaperone carries the polynucleotide. Further, inembodiments utilizing a processive exonuclease to feed individual basesthrough the nanopore, the kinetics of the exonuclease may be modifiedbased on the known biochemical characteristics of the exonuclease.

In diagnostic sequencing applications, it may be necessary only toprovide sequence data for a small fragment of DNA, but do so in anextremely accurate sequencing process. For such applications, shortertarget segments may be employed, thus permitting a higher level ofredundancy by sequencing multiple times around a smaller circulartemplate, where such redundancy provides the desired accuracy. Thus, insome cases, the double stranded target segment may be much shorter,e.g., from 10 to 200, from 20 to 100 or from 20 to 50 or from 20 to 75bases in length. For purposes of the foregoing, the length of the targetsegment in terms of bases denotes the length of one strand of the doublestranded segment. In such applications, various methods for intermittentdetection described herein may be used to analyze the sequence of thetemplate, thereby targeting the sequence data to the portion(s) of thetemplate of particular interest to the diagnostician, and/or improvingvarious aspects of the reaction performance, e.g., by virtue of thereduction of photo-induced damage to one or more reaction components.

It is to be understood that the above description is intended to beillustrative and not restrictive. It readily should be apparent to oneskilled in the art that various embodiments and modifications may bemade to the invention disclosed in this application, including but notlimited to combinations of various aspects of the invention, withoutdeparting from the scope and spirit of the invention. The scope of theinvention should, therefore, be determined not with reference to theabove description, but should instead be determined with reference tothe appended claims, along with the full scope of equivalents to whichsuch claims are entitled. All publications mentioned herein are citedfor the purpose of describing and disclosing reagents, methodologies andconcepts that may be used in connection with the present invention.Nothing herein is to be construed as an admission that these referencesare prior art in relation to the inventions described herein. Throughoutthe disclosure various patents, patent applications and publications arereferenced. Unless otherwise indicated, each is incorporated byreference in its entirety for all purposes.

Although described in some detail for purposes of illustration, it willbe readily appreciated that a number of variations known or appreciatedby those of skill in the art may be practiced within the scope ofpresent invention. Unless otherwise clear from the context or expresslystated, any concentration values provided herein are generally given interms of admixture values or percentages without regard to anyconversion that occurs upon or following addition of the particularcomponent of the mixture. To the extent not already expresslyincorporated herein, all published references and patent documentsreferred to in this disclosure are incorporated herein by reference intheir entirety for all purposes.

The following non-limiting examples are provided to further illustratethe invention.

VI. Examples of Intermittent Illumination of a Single MoleculeSequencing-by-Synthesis Reaction Example I

A nucleic acid template was provided that comprised a double-strandedregion and two single-stranded linker portions at each end. The firstlinker portion connected the 3′ end of the sense strand with the 5′ endof the antisense strand, and the second linker portion connected the 3′end of the antisense strand with the 5′ end of the sense strand. Thistemplate was designed to form a single-stranded circle of approximately500 bases when the double-stranded region was opened (e.g., by heatdenaturization, helicase activity, etc.), and is sometimes referred toas a SMRTbell™ template. A plurality of this nucleic acid template wasincubated with polymerases, primers, and other reaction components toallow formation of polymerase-template complexes. (See, e.g., Korlach,J., et al. (2008) Nucleosides, Nucleotides and Nucleic Acids,27:1072-1083; and Eid, J. (2009) Science 323:133-138.) The complexeswere immobilized in zero-mode waveguides in a reaction mixturecontaining all necessary buffer and nucleotide analog components forcarrying out sequencing-by-synthesis reactions with the exception of acognate starting base and a metal dication. A Smith-Waterman algorithmwas used to perform the alignment of the known sequence of the templatewith the sequence reads generated in the reaction, and the positions ofthe sequence reads is graphically illustrated in FIG. 8.

Acquisition of the data shown in FIG. 8 was collected as follows.Illumination of the array of zero-mode waveguides was initiated withlaser excitation (532 nm and 641 nm laser lines) at t=−5 seconds, andthe missing cognate starting base and metal dication (manganese metal)were added at t=0 seconds to simultaneously initiate thesequencing-by-synthesis reactions in all zero-mode waveguides. Thereactions were monitored under illumination for 120 seconds at whichtime the illumination was removed; the sequencing reads generated duringthat stage of the reaction are shown in FIG. 8A as a function of thetemplate position to which each read maps. At 295 seconds illuminationwas resumed and data acquisition was reinitiated at 300 seconds andmaintained for another 120 second interval; the sequencing reads duringthis second illuminated period are shown in FIG. 8B. At 595 secondsillumination was resumed and data acquisition was reinitiated at 600seconds and maintained for another 120 second interval; the sequencingreads during this third illuminated period are shown in FIG. 8C.

As expected, the longer the amount of time before the sequence data iscollected (that is, the later the illuminated period), the further intothe template the alignments shift, and this shift is a rough function oftime since initiation of the reaction. Further, the distribution ofsequence reads generated during each subsequent illuminated periodbecomes more dispersed than the previous illuminated period(s). Further,due to the circular nature of the template, FIG. 8C clearly shows thatsome polymerases have passed completely around the substrate and arebeginning to generate sequence reads from a second pass around thetemplate, thereby generating redundant sequence information for a singletemplate nucleic acid.

Example II

As in Example I, a SMRTbell™ template was used. For templates of definedsequence, PCR was used to generate 3 or 6 kb DNA inserts for thedouble-stranded region in the SMRTbell™ templates using a standard PCRmethodology. For genomic and other biological samples, a DNAfragmentation protocol was used that generates DNA fragments distributedaround 3 or 6 kb. Generation of fragments in these ranges was done usinga HydroShear® (Genomic Solutions®) device with settings recommended bythe manufacturer. The random genomic DNA fragments were enzymaticallytreated to generate blunt ends. Both the PCR products and randomlygenerated DNA fragments were phosphorylated and then immediately putinto a ligation reaction with a blunt hairpin adapter. The products werepurified through two size selection steps using reduced volumes ofAMPure® magnetic beads (Agencourt®) to remove hairpin dimers and othershort products. (Fabrication of SMRTbell™ templates is further describedelsewhere herein.)

The system components used for polynucleotide sequencing usingintermittent detection are comparable to single-molecule sequencingapplications under constant illumination, which are described, e.g., inEid, et al. (2009) Science 323:133-138. Specifically, the immobilizationand sequencing buffer compositions, nucleotide analogs identity andconcentration, polymerase, ZMWs, surface treatment and instrumentationwere identical to the standard methodology. Modifications to theSMRTbell™ template DNA and polymerase binding and immobilization anddata acquisition protocols are as follows.

A binding solution was prepared by incubation of 3 or 6 kb DNA SMRTbell™templates (1-10 nM) with a 10-fold excess of DNA polymerase (10-100 nM,respectively) in 10 mM MOPS (pH 7.5), 10 mM KOAc, 100 mM DTT & 0.05%Tween-20 for 2 hours at 30° C., followed by 1 hour at 37° C. andsubsequent storage at 4° C. prior to immobilization on the ZMWs.Immediately prior to immobilization, the binding solution was diluted inthe standard immobilization solution (50 mM MOPS (pH 7.5), 75 mM KOAc, 5mM DTT, 0.05% Tween-20) to the desired final concentration, typically0.1 to 1 nM, and incubated for 30 to 60 minutes at 22° C.Post-immobilization chip preparation and sequencing initiation wereidentical to the standard methods.

The data acquisition protocol was similar to the standard applicationwith coordinated modifications to the collection timing and ZMWpositioning. In the standard acquisition procedure, a single longacquisition (˜10 minutes) is performed for each ZMW. In the intermittentillumination acquisition procedure, multiple short acquisitions (˜3minutes) of sequence reads (also termed “strobe reads”) were performedfor each ZMW (during “detection periods”) with an interval between eachacquisition period during which no acquisition of sequence reads wasperformed (“non-detection period”). The duration of the interval betweeneach acquisition of sequence reads was determined based upon a desireddistance (i.e., number of nucleotide positions) between each sequence(or strobe) read, the polymerization rate of the polymerase, and theSMRTbell™ template insert size.

SMRTbell™ templates were generated as described above for AC223433, afosmid clone comprising a sequence of an approximately 40 kb region ofHomo sapiens chromosome 15. The reference sequences used to map thesequence reads generated in the sequencing reactions were the publicallyavailable sequences of Homo sapiens chromosome 15 (Hg18; NCBI Build36.1) and fosmid AC223433 (NCBI GenBank accession number). Table 1 showsthe number of statistically significantly mapped sequence reads forseveral types of intermittent illumination sequencing reactions. Thenumber of mappable “looks” is equivalent to the number of mappablesequence reads generated during detection periods for a single templatemolecule. For example, a “mapped 1-look read” means, for a singletemplate molecule, only a single detection period generated a sequenceread that could be mapped to the reference sequence.

TABLE 1 Summary of Sequencing Results Mapped Mapped Mapped MappedMapping 1-look 2-look 3-look 4-look Reference reads reads reads readsHuman 13834 1289 127 4 chr15 (Hg18) Fosmid 15253 1571 158 5

Deviations in the expected time span for a set of sequencing reads froma single sequencing reaction are indicative of genomic events such asgenomic rearrangements, e.g., insertions, deletions, etc. FIGS. 10 and11 illustrate this point. Specifically, the time and distance travelledalong the template (based upon the reference sequence) by the polymerasewas computed within and between the sequence reads generated during thedetection (illuminated) periods. These calculations were used to detectunexpected variations, indicating possible genomic events in thetemplate as compared to the reference sequence. FIG. 10 provides a plotthat illustrates the normalized average time it took for the polymeraseto traverse a region of the template based on the length of that regionin the Homo sapiens chromosome 15 reference sequence. The sequence readsare fit to a diagonal having a slope equal to the average speed forsequencing reads. Deviations from the regressed diagonal indicategenomic events (for example, structural variants), and the slope of thesequence reads around such deviations indicate the relative size of thegenomic event (e.g., in the case of insertions/deletions). For example,if the time for the polymerase to traverse a region was unexpectedlylong, this indicated the polymerase actually traversed a longer regionthan was expected based on the reference sequence. The two distinctoff-diagonal deviations (upper right hand corner) with higher slopeindicated that an insertion had occurred in the reference sequence, andthis was verified by comparison to the known fosmid sequence.

FIG. 11 shows the average time it took the polymerase to traverse thetemplate. For each mapped read, starting and ending times and positionswere determined and used to compute the distance traversed by thepolymerase between sequence reads. Based on these determinations, anaverage time across any particular region of the human referencesequence was computed. Regions that were traversed by the polymerasemore slowly have peaks of higher ΔT, and were indicative of insertionsin the template relative to the Homo sapiens chromosome 15 referencesequence. The insertions identified were the same insertions identifiedabove.

Intermittent illumination-based sequencing reactions across fosmidsequence AC223433 showed significant sequence read coverage across theinsertion events. The distribution of the physical coverage is shown inFIG. 12, which illustrates examples of three-look strobes (i.e.,sequencing reactions having three detection/illuminated periods) thatspan or intersect the insertion events. FIG. 12A shows the mapping ofthe strobe sequence reads to the Homo sapiens chromosome 15 referencesequence, where the sequence reads generated from the insert sequencesin the template are excluded. Arrows indicate the locations of theinsertions. FIG. 12B shows a similar mapping with the sequence readsgenerated from the insert sequences indicated with brackets. A number ofsequence reads flank the insertions, connect the two insertions, orclarify sequence within (or at the boundaries of) the insertionsequences. Such flanking and connecting sequence reads are useful forpredicting and detecting genomic events, anchoring them to genomicreferences, and scaffolding for de novo assembly of novel sequences. Inparticular, there are 30 and 38 “3-look” reads that intersect the tworegions of insertion of (1192 bp and 6879 bp, respectively). Thesesequence reads facilitated mapping of the insertions to the humanreference sequence, which would have been extremely difficult, if notimpossible, with commercially available short-read sequencingtechnologies. Further the sequence of the smaller insertion was a highlyrepetitive sequence, which would also have made mapping difficult withcertain short-read technologies.

FIG. 13 illustrates the sequence coverage obtained across the fosmidsequence, showing all two-, three-, and four-look strobe sequence readsspanning the sequence that are mappable to the known AC223433 fosmidsequence.

A consensus sequence was derived from the set of mappable sequence readsgenerated in these sequencing reactions. Strobe sequence reads werecombined with sequence reads generated under constant illumination andassembled based on the human reference sequence (Hg18). High qualityreads surrounding the (suspected) insertion sites, as well as highquality reads that did not map to the reference sequence, were extractedand assembled with a “de novo” greedy suffix tree assembler; theresulting contigs were mapped to the Hg18 reference sequence. Contigsspanning the (suspected) insertion sites were identified and fed backinto the “de novo” assembler, and the resulting contigs were manuallyedited using standard techniques and placed back into the derivedreference guided assembly. The final consensus sequence was a hybrid ofa reference guided assembly and attempts at de novo assembly of novelinsert sequences. Alignments to reference sequences were performed andplotted. FIG. 14 provides a sequence dot plot for an alignment between asequence assembly produced as described above and the fosmid referencesequence, and this plot confirmed a high degree of alignment between thetwo sequences. This dot plot was generated using Gepard 1.21 (“GEnomePAir—Rapid Dotter,” available from the Munich Information Center forProtein Sequences (MIPS)) with a word size of 7. Nucleic acid dot plotsare widely used in the art and are further described, e.g., in Krumsieket al. (2007) Bioinformatics 23(8):1026-8; Maizel et al. (1981) ProcNatl Acad Sci USA 78:7665; Pustell, et al. (1982) Nucleic Acids Res10:4765; and Quigley, et al. (1984) Nucleic Acids Res 12:347, all ofwhich are incorporated herein by reference in their entireties for allpurposes.

1.-18. (canceled)
 19. A method of controlling the progress of apolymerase on a template nucleic acid comprising: providing a templatenucleic acid comprising a duplex fragment inserted between two hairpinlinkers, wherein one or both of the hairpin linkers comprise a stop orpause point and wherein at least one of the hairpin linkers comprises aprimer binding site; providing nucleotides, a primer complementary tothe at least one primer binding site, and a polymerase enzyme underconditions whereby nucleotides are incorporated by the polymerase enzymein a template-directed primer extension reaction, wherein the primerextension reaction is stopped or paused when the polymerase enzymeencounters a stop or pause point.
 20. The method of claim 19 wherein theone or both of the hairpin adaptors comprises a pause point, and themethod further comprises reinitiating the extension reaction after theextension reaction is paused.
 21. The method of claim 19 wherein thestop or pause point comprises a blocking moiety.
 22. The method of claim19 wherein the stop or pause point comprises a photolabile group. 23.The method of claim 19 wherein the stop or pause point comprises astrand binding moiety.
 24. The method of claim 19 wherein the stop orpause point comprises a non-native nucleotide.
 25. The method of claim19 wherein the stop or pause point comprises an abasic site.
 26. Themethod of claim 25, wherein providing the template nucleic acidcomprises treating the template nucleic acid with Uracil DNA glycosylaseto introduce the abasic site at a site-specific deoxyuracil of one orboth of the hairpin linkers.
 27. The method of claim 25 wherein one setof conditions are provided whereby the primer extension reaction isstopped at the abasic site, and another set of conditions are providedwhereby the extension reaction continues past the abasic site.
 28. Themethod of claim 27 wherein the another set of conditions comprise theuse of pyrene.
 29. The method of claim 19, wherein the stop or pausepoint is a site that causes a permanent cessation of nascent strandsynthesis.
 30. The method of claim 19 wherein the stop or pause pointcomprises a specific sequence to which a blocking group will bind. 31.The method of claim 30 wherein the blocking group is a reversibly boundblocking group
 32. The method of claim 31 wherein the reversibly boundblocking group is removed and the primer extension reaction isreinitiated.
 33. The method of claim 31 wherein the reversibly boundblocking group comprises a protein.
 34. The method of claim 31 whereinthe reversibly bound blocking group comprises a lac repressor proteinand the specific sequence comprises a lac repressor recognitionsequence.
 35. The method of claim 19 wherein the stop or pause pointcomprises a pyrimidine dimer
 36. The method of claim 24 wherein thenon-native nucleotide is produced by a modification enzyme.
 37. Themethod of claim 36 wherein the modification enzyme comprises aglycosylase, a methylase, or a nuclease.
 38. The method of claim 19wherein the duplex fragment comprises genomic DNA.